Layer mapping method and data transmission method for MIMO system

ABSTRACT

A method for indicating a combination between a codeword and a layer in a MIMO communication, system, a layer mapping method, and a data transmission method using the same are disclosed. A minimum number of codeword-layer mapping combinations from among all available combinations based on the’ numbers of all codewords and all layers are pre-defined in consideration of a ratio of a codeword to a layer, a reception performance of a receiver, and reduction of combinations, so that a data transmission method using the predefined combinations is implemented. If a.specific one codeword is mapped to at least two layers, a diversity gain can be acquired.

This application claims priority to International Application No.PCT/KR2008/000074 filed on Jan. 7, 2008, which claims priority to KoreanPatent Application No. 10-2007-0001353 filed on Jan. 5, 2007, KoreanPatent Application No. 10-2007-0002673 filed on Jan. 9, 2007, KoreanPatent Application No. 10-2007-0060166 filed on Jun. 19, 2007, KoreanPatent Application No. 10-2007-0072260 filed on Jul. 19, 2007 and KoreanPatent Application No. 10-2007-0080823 filed on Aug. 10, 2007, all ofwhich are incorporated by reference, as if fully set forth herein.

TECHNICAL FIELD

The present invention relates to a Multi-Input Multi-Output (MIMO)communication system, and more particularly to a method for displaying acombination between a codeword and a layer in a MIMO communicationsystem, a layer mapping method and a data transmission method using thesame.

BACKGROUND ART

A conventional MIMO technology will hereinafter be described in detail.

In brief, the MIMO technology is an abbreviation of the Multi-InputMulti-Output technology. The MIMO technology uses multiple transmission(Tx) antennas and multiple reception (Rx) antennas to improve theefficiency of Tx/Rx data, whereas a conventional art has generally useda single transmission (Tx) antenna and a single reception (Rx) antenna.In other words, the MIMO technology allows a transmitter or receiver ofa wireless communication system to use multiple antennas (hereinafterreferred to as a multi-antenna), so that the capacity or performance canbe improved. For the convenience of description, the term “MIMO” canalso be considered to be a multi-antenna technology.

In more detail, the MIMO technology is not dependent on a single antennapath to receive a single total message, collects a plurality of datapieces received via several antennas, and completes total data. As aresult, the MIMO technology can increase a data transmission rate at agive channel condition, or can increase a system performance at aspecific data transmission rate.

The next-generation mobile communication technology requires a datatransmission rate higher than that of a conventional mobilecommunication technology, so that it is expected that the effective MIMOtechnology is requisite for the next-generation mobile communicationtechnology. Under this situation, the MIMO communication technology isthe next-generation mobile communication technology capable of beingapplied to mobile communication terminals or base stations, and canextend the range of a data communication range, so that it can overcomethe limited amount of transfer data of other mobile communicationsystems due to a variety of limited situations.

Among a variety of technologies capable of improving the transmissionefficiency of data, the MIMO technology can greatly increase an amountof communication capacity and Tx/Rx performances without allocatingadditional frequencies or increasing an additional power. Due to thesetechnical advantages, most companies or developers have intensively paidattention to this MIMO technology.

FIG. 1 is a block diagram illustrating a conventional MIMO communicationsystem.

Referring to FIG. 1, if the number of transmission (Tx) antennasincreases to N_(T), and at the same time the number of reception (Rx)antennas increases to N_(R), a theoretical channel capacity of the MIMOcommunication system increases in proportion to the number of antennas,so that a transmission rate and a frequency efficiency can greatlyincrease.

In this case, the transmission rate acquired by the increasing channelcapacity is equal to the multiplication of a maximum transmission rate(Ro) acquired when a single antenna is used and a rate increment (Ri),and can theoretically increase. The rate increment (Ri) can berepresented by the following equation 1:R _(i)=min(N _(T) ,N _(R))  [Equation 1]

For example, provided that the MIMO system uses four Tx antenna and fourRx antennas, this MIMO system can theoretically acquire a hightransmission rate which is four times higher than that of a singleantenna system.

After the above-mentioned theoretical capacity increase of the MIMOsystem has been demonstrated in the mid-1990s, many developers areconducting intensive research into a variety of technologies which cansubstantially increase a data transmission rate using the theoreticalcapacity increase. Some of them have been reflected in a variety ofwireless communication standards, for example, a third-generation mobilecommunication or a next-generation wireless LAN, etc.

A variety of MIMO-associated technologies have been intensivelyresearched by many companies or developers, for example, research intoan information theory associated with a MIMO communication capacitycalculation under various channel environments or multiple accessenvironments, research into a wireless channel measurement and modelingof the MIMO system, and research into a space-time signal processingtechnology.

The above-mentioned MIMO technology can be classified into two types: aspatial diversity scheme and a spatial multiplexing scheme. The spatialdiversity scheme increases transmission reliability using symbolspassing various channel paths. The spatial multiplexing schemesimultaneously transmits a plurality of data symbols via a plurality ofTx antennas, so that it increases a transmission rate of data. Inaddition, the combination of the spatial diversity scheme and thespatial multiplexing scheme has also been recently developed to properlyacquire unique advantages of the two schemes.

Details of the spatial diversity scheme, the spatial multiplexingscheme, and the combination thereof will hereinafter be described.

Firstly, the spatial diversity scheme will hereinafter be described. Byand large, the spatial diversity scheme is divided into two types: aspace-time block code scheme and a space-time Trellis code scheme whichcan simultaneously uses a diversity gain and a coding gain. Generally, abit error ratio (BER) improvement performance and a code-generationdegree of freedom of the space-time Trellis code scheme are superior tothose of the space-time block code scheme, whereas the calculationcomplexity of the space-time block code scheme is higher than that ofthe space-time Trellis code scheme.

The above-mentioned spatial diversity gain corresponds to the product ormultiplication of the number (N_(T)) of Tx antennas and the number(N_(R)) of Rx antennas, as denoted by N_(T)×N_(R).

Secondly, the spatial multiplexing scheme will hereinafter be described.The spatial multiplexing scheme is adapted to transmit different datastreams via individual Tx antennas. In this case, a receiver mayunavoidably generate mutual interference between data piecessimultaneously transmitted from a transmitter. The receiver removes thismutual interference from the received data using a proper signalprocessing technique, so that it can receive the desired data having nointerference. In order to remove noise or interference from the receiveddata, a maximum likelihood receiver, a ZF (Zero Forcing) receiver, aMMSE (Minimum Mean Square Error) receiver, a D-BLAST, or a V-BLAST maybe used. Specifically, if a transmitter can recognize channelinformation, a Singular Value Decomposition (SVD) scheme may be used toremove the interference perfectly.

Thirdly, the combination of the spatial diversity scheme and the spatialmultiplexing scheme will hereinafter be described. Provided that only aspatial diversity gain is acquired, the performance-improvement gain isgradually saturated in proportion to an increasing diversity order. As aresult, a variety of schemes capable of acquiring all theabove-mentioned two gains simultaneously while solving theabove-mentioned problems have been intensively researched by manycompanies or developers, for example, a double-STTD scheme and aspace-time BICM (STBICM) scheme.

A mathematical modeling of a communication method for use in theabove-mentioned MIMO system will hereinafter be described in detail.

Firstly, as can be seen from FIG. 1, it is assumed that N_(T) Txantennas and N_(R) Rx antennas exist.

In the case of a transmission (Tx) signal, a maximum number oftransmission information pieces is N_(T) under the condition that N_(T)Tx antennas are used, so that the Tx signal can be represented by aspecific vector shown in the following equation 2:s=[s ₁ ,s ₂ , . . . ,s _(N) _(T) ]^(T)  [Equation 2]

The individual transmission information pieces (s₁, s₂, . . . s_(N) _(T)) may have different transmission powers. In this case, if theindividual transmission powers are denoted by (P₁, P₂, . . . , P_(N)_(T) ), transmission information having an adjusted transmission powercan be represented by a specific vector shown in the following equation3:ŝ=[ŝ ₁ ,ŝ ₂ , . . . ,ŝ _(N) _(T) ]^(T) =[P ₁ s ₁ ,P ₂ s ₂ , . . . ,P_(N) _(T) s _(N) _(T) ]^(T)   [Equation 3]

In Equation 3, using a diagonal matrix of a transmission power P, ŝ canbe represented by the following equation 4:

$\begin{matrix}{\hat{s} = {{\begin{bmatrix}P_{1} & \; & \; & 0 \\\; & P_{2} & \; & \; \\\; & \; & \ddots & \; \\0 & \; & \; & P_{N_{T}}\end{bmatrix}\begin{bmatrix}s_{1} \\s_{2} \\\vdots \\s_{N_{T}}\end{bmatrix}} = {P\; s}}} & \lbrack {{Equation}\mspace{14mu} 4} \rbrack\end{matrix}$

The information vector ŝ having an adjusted transmission power ismultiplied by a weight matrix (W), so that N_(T) transmission (Tx)signals (x₁, x₂, . . . x_(N) _(T) ) to be actually transmitted areconfigured. In this case, the weight matrix is adapted to properlydistribute Tx information to individual antennas according to Tx-channelsituations. The above-mentioned Tx signals (x₁, x₂, . . . , x_(N) _(T) )can be represented by the following equation 5 using the vector (x):

$\begin{matrix}{x = {\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{i} \\\vdots \\x_{N_{T}}\end{bmatrix} = {{\lbrack {\begin{matrix}w_{11} \\w_{21} \\\vdots \\w_{i\; 1} \\\vdots \\w_{N_{T}1}\end{matrix}\begin{matrix}w_{12} \\w_{22} \\\; \\w_{i\; 2} \\\; \\w_{N_{T}2}\end{matrix}\begin{matrix}\ldots \\\ldots \\\ddots \\\ldots \\\ddots \\\ldots\end{matrix}\begin{matrix}w_{1N_{T}} \\w_{2N_{T}} \\\; \\w_{i\; N_{T}} \\\; \\w_{N_{T}N_{T}}\end{matrix}} \rbrack\begin{bmatrix}{\hat{s}}_{1} \\{\hat{s}}_{2} \\\vdots \\{\hat{s}}_{j} \\\vdots \\{\hat{s}}_{N_{T}}\end{bmatrix}} = {{W\hat{s}} + {WPs}}}}} & \lbrack {{Equation}\mspace{14mu} 5} \rbrack\end{matrix}$

In Equation 5, w_(ij) is a weight between the i-th Tx antenna and thej-th Tx information, and W is a matrix indicating the weight w_(ij). Thematrix W is called a weight matrix or a precoding matrix.

The above-mentioned Tx signal (x) can be considered in different waysaccording to two cases, i.e., a first case in which the spatialdiversity is used and a second case in which the spatial multiplexing isused.

In the case of using the spatial multiplexing, different signals aremultiplexed and the multiplexed signals are transmitted to adestination, so that elements of the information vector (s) havedifferent values. Otherwise, in the case of using the spatial diversity,the same signal is repeatedly transmitted via several channel paths, sothat elements of the information vector (s) have the same value.

Needless to say, the combination of the spatial multiplexing scheme andthe spatial diversity scheme may also be considered. In other words, thesame signal is transmitted via three Tx antennas according to thespatial diversity scheme, and the remaining signals are spatiallymultiplexed and then transmitted to a destination.

Next, if N_(R) Rx antennas are used, Rx signals (y₁, y₂, . . . , y_(N)_(R) ) of individual antennas can be represented by a specific vector(y) shown in the following equation 6:y=[y ₁ ,y ₂ , . . . ,y _(N) _(R) ]^(T)  [Equation 6]

If a channel modeling is set up in the MIMO communication system,individual channels can be distinguished from each other according toTx/Rx antenna indexes. A specific channel from a Tx antenna (j) to an Rxantenna (i) is denoted by h_(ij). In this case, it should be noted thatthe first index of the channel h_(ij) indicates an Rx-antenna index andthe second means a Tx-antenna index.

Several channels are tied up, so that they are displayed in the form ofa vector or matrix. An exemplary vector is as follows.

FIG. 2 shows channels from N_(T) Tx antennas to an Rx antenna (i).

Referring to FIG. 2, the channels from the N_(T) Tx antennas to the Rxantenna (i) can be represented by the following equation 7:h _(i) ^(T) =[h _(i1) ,h _(i2) , . . . ,h _(iN) _(T) ]  [Equation 7]

If all channels from the N_(T) Tx antennas to N_(R) Rx antennas aredenoted by the matrix composed of Equation 7, the following equation 8is acquired:

$\begin{matrix}{H = {\begin{bmatrix}h_{1}^{T} \\h_{2}^{T} \\\vdots \\h_{i}^{T} \\\vdots \\h_{N_{R}}^{T}\end{bmatrix} = \lbrack {\begin{matrix}h_{11} \\h_{21} \\\vdots \\h_{i\; 1} \\\vdots \\h_{N_{R}1}\end{matrix}\begin{matrix}h_{12} \\h_{22} \\\; \\h_{i\; 2} \\\; \\h_{N_{R}2}\end{matrix}\begin{matrix}\ldots \\\ldots \\\ddots \\\ldots \\\ddots \\\ldots\end{matrix}\begin{matrix}h_{1N_{T}} \\h_{2N_{T}} \\\; \\h_{i\; N_{T}} \\\; \\h_{N_{R}N_{T}}\end{matrix}} \rbrack}} & \lbrack {{Equation}\mspace{14mu} 8} \rbrack\end{matrix}$

An Additive White Gaussian Noise (AWGN) is added to an actual channelwhich has passed the channel matrix H shown in Equation 8. The AWGN (n₁,n₂, . . . , n_(N) _(R) ) added to each of N_(R) Rx antennas can berepresented by a specific vector shown in the following equation 9:n=[n ₁ ,n ₂ , . . . ,n _(N) _(R) ]^(T)  [Equation 9]

By the above-mentioned modeling method of the Tx signal, Rx signal, andchannels including AWGN, each MIMO communication system can berepresented by the following equation 10:

$\begin{matrix}{y = {\begin{bmatrix}y_{1} \\y_{2} \\\vdots \\y_{i} \\\vdots \\y_{N_{R}}\end{bmatrix} = {{{\lbrack {\begin{matrix}h_{11} \\h_{21} \\\vdots \\h_{i\; 1} \\\vdots \\h_{N_{R}1}\end{matrix}\begin{matrix}h_{12} \\h_{22} \\\; \\h_{i\; 2} \\\; \\h_{N_{R}2}\end{matrix}\begin{matrix}\ldots \\\ldots \\\ddots \\\ldots \\\ddots \\\ldots\end{matrix}\begin{matrix}h_{1N_{T}} \\h_{2N_{T}} \\\; \\h_{i\; N_{T}} \\\; \\h_{N_{R}N_{T}}\end{matrix}} \rbrack\begin{bmatrix}x_{1} \\x_{2} \\\vdots \\x_{j} \\\vdots \\x_{N_{T}}\end{bmatrix}} + \begin{bmatrix}n_{1} \\n_{2} \\\vdots \\n_{i} \\\vdots \\n_{N_{R}}\end{bmatrix}} = {{Hx} + n}}}} & \lbrack {{Equation}\mspace{14mu} 10} \rbrack\end{matrix}$

The above-mentioned description has disclosed that the MIMOcommunication system is applied to a single user. However, the MIMOcommunication system may also be applied to several users, so that itcan acquire a multi-user diversity. A detailed description of themulti-user diversity will hereinafter be described.

The fading channel is a major cause of deterioration of a performance ofa wireless communication system. A channel gain value is changedaccording to time, frequency, and space. The lower the channel gainvalue, the lower the performance. A representative method for solvingthe above-mentioned fading problem is a diversity. This diversity usesthe fact that there is a low probability that all independent channelshave low gain values. A variety of diversity methods can be applied tothe present invention, and the above-mentioned multi-user diversity isconsidered to be one of them.

If several users are present in a cell, channel gain values ofindividual users are stochastically independent of each other, so thatthe probability that all the users have low gain values is very low. Ifa Node-B has sufficient transmission (Tx) power and several users arepresent in a cell, it is preferable that all channels be allocated to aspecific user having the highest channel gain value to maximize a totalchannel capacity according to the information theory. The multi-userdiversity can be classified into three kinds of diversities, i.e., atemporal multi-user diversity, a frequency multi-user diversity, and aspatial multi-user diversity.

The temporal multi-user diversity is adapted to allocate a channel to aspecific user having the highest gain value when a channel situationchanges with time.

The frequency multi-user diversity is adapted to allocate asub-carrier(s) to a specific user having the highest gain value in eachfrequency band in a frequency multi-carrier system such as an OrthogonalFrequency Division Multiplexing (OFDM) system.

If a channel situation slowly changes with time in another system whichdoes not use the multi-carrier, the user having the highest channel gainvalue will monopolize the channel for a long period of time, other usersare unable to communicate with each other. In this case, in order to usethe multi-user diversity, there is a need to induce the channel tochange.

Next, the spatial multi-user diversity uses different channel gainvalues of users according to space types. An implementation example ofthe spatial multi-user diversity is a Random BeamForming (RBF) method.This RBF method performs beamforming with a predetermined weight usingmultiple antennas (i.e., multi-antenna) to induce the change of channel,and uses the above-mentioned spatial multi-user diversity.

The multi-user MIMO scheme which uses the multi-user diversity as themulti-antenna scheme will hereinafter be described in detail.

According to the multi-user multi-antenna scheme, the number of usersand the number of antennas of each user can be combined with each otherin various ways at transmission/receivers.

The multi-user MIMO scheme is classified into a downlink method (i.e., aforward-link method) and an uplink method (i.e., a reverse-link method),and detailed descriptions of the downlink and uplink methods willhereinafter be described. In this case, the downlink indicates that asignal is transmitted from a Node-B to several user equipments (UEs),and the uplink indicates that several UEs transmit a signal to theNode-B.

The downlink in MIMO can be generally categorized into two kinds ofsignal reception methods: The first reception method enables a singleuser (i.e., a single UE) to receive a desired signal via a total ofN_(R) antennas, and the second reception method enables each of theN_(R) UEs to receive a desired signal via a single antenna. If required,a combination of the first and second reception methods may also be madeavailable for the present invention. In other words, some UEs may use asingle Rx antenna, or some other UEs may use three Rx antennas. Itshould be noted that a total number of Rx antennas in all combinationsis maintained at N_(R). This case is generally called a MIMO BroadcastChannel (BC) or a Space Division Multiple Access (SDMA).

The uplink in MIMO can be generally classified into two kinds of signaltransmission methods: The first transmission method enables a single UEto transmit a transmission method enables each of the N_(T) UEs totransmit a desired signal via a single antenna. If required, acombination of the first and second transmission methods may also bemade available for the present invention. In other words, some UEs mayuse a single Tx antenna, or some other UEs may use three Tx antennas. Itshould be noted that a total number of Tx antennas in all combinationsis maintained at N_(T). This case is generally called a MIMO MultipleAccess Channel (MAC).

The uplink and the downlink are symmetrical to each other, so that amethod for use in one of them may also be used for the other one.

For the convenience of description and better understanding of thepresent invention, although the following description will basicallydescribe the MIMO BC, it should be noted that the method of the presentinvention be also used for the MIMO MAC.

FIG. 3A is a conceptual diagram illustrating a single-user MIMOcommunication system. FIG. 3B is a conceptual diagram illustrating amulti-user MIMO communication system.

For the convenience of description, FIGS. 3A and 3B assume the use of adownlink.

The single-user MIMO communication system shown in FIG. 3A includes atransmitter (i.e., Node-B) equipped with multiple antennas (i.e.,multi-antenna) and a receiver (i.e., UE) equipped with multipleantennas. In this case, if a signal (x) to be transmitted from thetransmitter is multiplied by a weight vector (W), and the multipliedresultant signal is transmitted via the multi-antenna, the presentinvention can acquire a maximum of channel capacity on the assumptionthat channel information has been correctly recognized.

In the meantime, the multi-user MIMO communication system shown in FIG.3B includes a plurality of Multiple Input Single Output (MISO) systems,each of which assigns a single antenna to each user. Therefore, themulti-user can maximize the channel capacity using a transmissionbeamforming in the same manner as in the single-user MIMO communicationsystem. In this case, the multi-user MIMO communication system mustconsider not only the channel information but also interference of eachuser, so that it requires a more complicated system than that of thesingle-user MIMO communication system. Therefore, the multi-user MIMOcommunication system must select a weight vector to minimize theinterference between users in the case of using the transmissionbeamforming.

The above-mentioned description can be numerically described as follows.

Firstly, the single-user environment, i.e., the single-user MIMOcommunication system, will hereinafter be described.

Provided that all transmission/receivers have fully recognized allchannel information, a singular value decomposition (SVD) H can berepresented by the following equation 11:H=UΣV ^(H)  [Equation 11]

where “H” is a singular value decomposition, U and V is a unitarymatrix, Σ is a diagonal matrix.

In this case, in order to acquire a maximum gain in the light of channelcapacity, the diagonal matrix V is selected by the weight matrix W, andU^(H) is multiplied by a reception signal (Y). If the resultant signalof the receiver is denoted by {tilde over (y)}, the following equation12 is acquired:W=Vy=Hx+n=UΣV ^(H) x+n=UΣV ^(H) Wŝ+n=UΣV ^(H) Vŝ+n=UΣŝ+n{tilde over (y)}=U ^(H) y=U ^(H) UΣŝ+U ^(H) n=Σŝ+ñ=ΣPs+ñ  [Equation 12]

where P is a transmission power matrix. The transmission power matrix Pcan be determined by a specific algorithm (well known as a water-fillingalgorithm) for acquiring the channel capacity. This water-fillingalgorithm is an optimum method for acquiring the channel capacity.

However, in order to perform the water-filling algorithm, all thetransmission/receivers must completely know all channel information.Therefore, in order to use the water-filling algorithm under themulti-user environment, each of all users must know not only his or herchannel information but also channel information of other users. Due tothis problem, in fact, it is almost impossible for the multi-user MIMOcommunication system to use the above-mentioned water-filling algorithm.

Next, the multi-user MIMO communication system will hereinafter bedescribed.

In this case, a representative optimum method for acquiring the channelcapacity is a Dirty Paper Coding (DPC) method, but this DPC method hashigh complexity. Also, there are other optimum methods for use in thepresent invention, for example, a Random BeamForming (RBF) and a ZeroForcing BeamForming (ZFBF). The above-mentioned RBF or ZFBF method mayhave a performance similar to the optimum performance acquired by theDPC method, if the number of users increases in the multi-userenvironment.

In the meantime, a codeword for use in the MIMO communication systemwill hereinafter be described.

A general communication system performs coding of transmissioninformation of a transmitter using a forward error correction code, andtransmits the coded information, so that an error experienced at achannel can be corrected by a receiver. The receiver demodulates areceived (Rx) signal, and performs decoding of forward error correctioncode on the demodulated signal, so that it recovers the transmissioninformation. By the decoding process, the Rx-signal error caused by thechannel is corrected.

Each of all forward error correction codes has a maximum-correctablelimitation in a channel error correction. In other words, if a reception(Rx) signal has an error exceeding the limitation of a correspondingforward error correction code, a receiver is unable to decode the Rxsignal into information having no error. Therefore, the receiver mustdetermine the presence or absence of an error in the decodedinformation. In this way, a specialized coding process for performingerror detection is required, separately from the forward errorcorrection coding process. Generally, a Cyclic Redundancy Check (CRC)code has been used as an error detection code.

The CRC method is an exemplary coding method for performing the errordetection. Generally, the transmission information is coded by the CRCmethod, and then the forward error correction code is applied to theCRC-coded information. A single unit coded by the CRC and the forwarderror correction code is generally called a codeword.

In the meantime, if several transmission information units areoverlapped and then received, the present invention can expectperformance improvement using an interference-cancellation receiver.There are many cases in the above-mentioned case in which severaltransmission information is overlapped and then received, for example, acase in which the MIMO technology is used, a case in which a multi-userdetection technology is used, and a case in which a multi-codetechnology is used. A brief description of the interference-cancellationstructure will be as follows.

According to the interference-cancellation structure, after firstinformation is demodulated/decoded from a total reception signal inwhich several information is overlapped, information associated with thefirst information is removed from the total reception signal. A secondsignal is demodulated/decoded by the resultant signal having no firstinformation removed from the reception signal. A third signal isdemodulated/decoded by the resultant signal having no first- andsecond-information removed from the first reception signal. A fourthsignal or other signal after the fourth signal repeats theabove-mentioned processes, so that the fourth or other signal isdemodulated/decoded. In this way, the above-mentioned method forcontinuously removing the demodulated/decoded signal from a receptionsignal to improve a performance of the next demodulating/decodingprocess is called a Successive Interference Cancellation (SIC) method.

In order to use the above-mentioned interference cancellation methodsuch as the SIC, the demodulated/decoded signal removed from thereception signal must have no error. If any error occurs in thedemodulated/decoded signal, an error propagation occurs so that anegative influence continuously affects all the demodulated/decodedsignals.

The above-mentioned interference cancellation technology can also beapplied to the MIMO technology. If several transmission informationpieces are overlapped/transmitted via multiple antennas, theabove-mentioned interference cancellation technology is required. Inother words, if the spatial multiplexing technology is used, eachtransmitted information is detected, and at the same time theinterference cancellation technology can be used.

However, as described above, in order to minimize the error propagationcaused by the interference cancellation, it is preferable that theinterference is selectively removed after determining the presence orabsence of an error in the demodulated/decoded signal. A representativemethod for determining the presence or absence of the error in eachtransmission information is the above-mentioned cyclic redundancy check(CRC) method. A unit of distinctive information processed by the CRCcoding is called a codeword. Therefore, a more representative method forusing the interference cancellation technology is a specific case inwhich several transmission information pieces and several codewords areused.

In the meantime, the number of rows and the number of columns of achannel matrix H indicating a channel condition is determined by thenumber of Tx/Rx antennas. In the channel matrix H, the number of rows isequal to the number (N_(R)) of Rx antennas, and the number of columns isequal to the number (N_(T)) of Tx antennas. Namely, the channel matrix His denoted by N_(R)×N_(T) matrix.

Generally, a matrix rank is defined by a smaller number between thenumber of rows and the number of columns, in which the rows and thecolumns are independent of each other. Therefore, the matrix rank cannotbe higher than the number of rows or columns. The rank of the channelmatrix H can be represented by the following equation 13:rank(H)≦min(N _(T) ,N _(R))  [Equation 13]

Another definition of the above-mentioned rank can be defined by thenumber of eigen values other than “0” when the matrix iseigen-value-decomposed. Similarly, if the rank is SVD-processed, therank may also be defined by the number of singular values other than“0”. Therefore, the physical meaning of the rank in the channel matrixmay be considered to be a maximum number of transmission times of agiven channel capable of transmitting different information.

For the convenience of description, it is assumed that each of differentinformation pieces transmitted via the MIMO technology is a transmissionstream or a stream. This stream may also be called a layer, so that thenumber of transmission streams cannot be higher than the channel rankequal to the maximum number of transmission times of the channel capableof transmitting different information.

If the channel matrix is H, this channel matrix H can be represented bythe following equation 14:# of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 14]

where “# of streams” is indicative of the number of streams.

In the meantime, it should be noted that a single stream may betransmitted via one or more antennas.

A method for matching the stream with the antenna can be describedaccording to the MIMO technology types.

In the case where a single stream is transmitted via several antennas,this case may be considered to be the spatial diversity scheme. In thecase where several streams are transmitted via several antennas, thiscase may be considered to be the spatial multiplexing scheme. Needlessto say, a hybrid scheme between the spatial diversity scheme and thespatial multiplexing scheme may also be made available.

The relationship between the codeword and the stream in the MIMOcommunication system will hereinafter be described in detail.

FIG. 4 is a block diagram illustrating the relationship between thecodeword and the stream in the MIMO communication system.

A variety of methods for matching the codeword with the stream are madeavailable. A general method from among the various methods generatescodeword(s), allows each codeword to enter a codeword-stream mappingmodule, matches the codeword received from the codeword-stream mappingmodule with the stream(s), and transmits the stream to thestream-antenna mapping module, so that the stream is transmitted via theTx antenna.

A part for determining the combination between the codeword and thestream is denoted by a bold solid line in FIG. 4.

Ideally, the relationship between the codeword and the stream can befreely determined. A single codeword may be divided into severalstreams, so that the divided streams are transmitted to a destination.Several codewords are serially integrated in one stream, so that thisstream including the codewords may be transmitted to a destination.

However, the above-mentioned serial-integration of several codewords maybe considered to be a kind of predetermined coding process, so that thepresent invention assumes that a single codeword is matched with one ormore streams of a real-meaningful combination. However, provided thatseveral streams are distinguished from each other without departing fromthe scope or spirit of the present invention, the present invention canalso be applied to the distinguished streams.

Therefore, for the convenience of description, the present inventionassumes that a single codeword is matched with one or more streams.Therefore, if all information is coded and then transmitted to adestination, the following equation 15 can be acquired:# of codewords≦# of streams  [Equation 15]

where “# of codewords” is the number of codewords, and “# of streams” isthe number of streams.

In conclusion, the above-mentioned equations 13 to 15 can be representedby the following equation 16:# of codewords≦# of streams≦rank(H)≦min(N _(T) ,N _(R))  [Equation 16]

By Equation 16, the following fact can be recognized. In other words, ifthe number of Tx/Rx antennas is limited, a maximum number of streams isalso limited. If the number of codewords is limited, a minimum number ofstreams is also limited.

By the above-mentioned relationship between the codeword and the stream,if the number of antennas is limited, the maximum number of codewords orstreams is limited, so that the limited number of codewords can becombined with the limited number of streams.

The above-mentioned combination between the codeword and the stream isrequired for both an uplink and a downlink.

For example, it is assumed that the MIMO technology is applied to thedownlink. In this case, a receiver must correctly be informed of acombination beforehand, which is used for the above-mentionedinformation transmission from among all combinations between thecodeword and the stream, so that the demodulating/decoding process ofthe information can be correctly performed.

Also, if control information is transmitted to the uplink, a preferredcombination from among various combinations between the codeword and thestream must also be recognized by a receiver. In more detail, in orderto implement the MIMO technology, a transmitter must recognize channeland status information of a receiver, so that the receiver must notifyvarious control information via the uplink.

For example, the receiver considers a variety of receiver states (e.g.,a measured channel or buffer status), and must notify a preferredcombination between a codeword and a stream, a channel quality indicator(CQI) corresponding to this preferred combination, and a precodingmatrix index (PMI) corresponding to the same. Needless to say, thecontents of detailed control information may be differently determinedaccording to the type of a used MIMO technology. However, theabove-mentioned fact in which the receiver must inform the uplink of thepreferred combination between the codeword and the stream isunchangeable.

For another example, if the MIMO technology is applied to the uplink,only a transmission link is changed to another link differently from theabove-mentioned example's description, and the remaining facts otherthan the change of the transmission link are equal to those of theabove-mentioned example, so that all combination between a codeword anda stream, a used combination, and a preferred combination must benotified.

If all the combinations between the codeword and the stream can beindicated by a small number of bits, control information can be moreeffectively transmitted to a destination. Therefore, there is needed amethod for effectively indicating the combination between the codewordand the stream.

DISCLOSURE Technical Problem

Accordingly, the present invention is directed to a layer mapping methodand a data transmission method for a MIMO system that substantiallyobviates one or more problems due to limitations and disadvantages ofthe related art.

An object of the present invention is to provide a method for rationallylimiting the number of combinations between a codeword and a stream, andreducing the number of bits of information indicating the number ofcombinations.

Another object of the present invention is to provide a method forlimiting the number of combinations between a codeword and a streamunder a multi-user MIMO communication system, reducing the number ofbits of information required for indicating the number of combinations,and providing an effective communication service.

Yet another object of the present invention is to provide a dataprocessing method for effectively dividing a single codeword into atleast two layers when data is transmitted via multiple Tx antennas in awireless communication system.

Yet another object of the present invention is to provide a dataprocessing method for transmitting data via multiple Tx antennas toreduce an influence of the fading phenomenon in a wireless communicationsystem.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

Technical Solution

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein, alayer mapping method for a spatial multiplexing in a Multi-InputMulti-Output (MIMO) system comprising: a) modulating a predetermined bitblock of each of at least one codeword, and generating a modulationsymbol stream for each codeword; and b) mapping a modulation symbol foreach of the at least one codeword to at least one layer according to onespecific combination from among predetermined mapping combinations,wherein, in each of the predetermined mapping combinations, the numberof layers to which a single codeword is mapped is limited to apredetermined number or below, when the predetermined number correspondsto a ratio acquired when the number of all layers is divided by thenumber of all codewords.

Preferably, the MIMO system uses a maximum of 4 layers and a maximum of2 codewords, and the predetermined mapping combination limits the numberof layers to which the single codeword is mapped to 2 or below.

Preferably, in the predetermined mapping combination, a combination, inwhich a single codeword is mapped to at least two layers when a maximumof one codeword is used, is removed from among all available mappingcombinations between the at least one codeword and the at least onelayer.

Preferably, in the predetermined mapping combination, a combination inwhich a single codeword is mapped to at least two layers when a maximumnumber of all layers is 2, is removed from among all available mappingcombinations between the at least one codeword and the at least onelayer.

Preferably, the predetermined mapping combination is limited to acombination in which one codeword with a lower index is mapped to asingle layer and the other codeword with a higher index is mapped to twolayers, when the two codewords are mapped to the three layers.

Preferably, the predetermined mapping combination is a combination inwhich combinations each of which has the same number of codewords andlayers are denoted by a single combination.

Preferably, the MIMO system uses a maximum of 4 layers and a maximum of2 codewords, and the predetermined mapping combination consists of afirst combination in which a single codeword is mapped to a singlelayer, a second combination in which two codewords are mapped to twolayers, respectively, a third combination in which two codewords aremapped to three layers, and a fourth combination in which two codewordsare mapped to four layers.

Preferably, the MIMO system uses a maximum of 4 layers and a maximum of2 codewords, and the predetermined mapping combination consists of afirst combination in which a single codeword is mapped to a singlelayer, a second combination in which two codewords are mapped to twolayers, respectively, a third combination in which two codewords aremapped to three layers, and a fourth combination in which two codewordsare mapped to four layers, and a fifth combination for supportingretransmission based on a Hybrid ARQ (HARQ) scheme.

Preferably, in the third combination, a first codeword from among thetwo codewords is mapped to a first layer from among the three layers,and a second codeword from among the two codewords is mapped to secondand third layers from among the three layers.

Preferably, in the fourth combination, a first codeword from among thetwo codewords is mapped to first and second layers from among the fourlayers, and a second codeword from among the two codewords is mapped tothird and fourth layers from among the four layers.

Preferably, in the layer mapping step b), if a specific one codeword ismapped to two layers according to the third or fourth combination fromamong the predetermined mapping combinations, modulation symbol streamsconstructing the specific one codeword are alternately mapped to the twolayers.

In another aspect of the present invention, there is provided a methodfor allowing a transmitter of a Multi-Input Multi-Output (MIMO) systemto transmit data via multiple transmission (Tx) antennas comprising: a)performing a channel encoding on a specific data block; b) modulating abit block formed by the channel-encoded data block, and generating amodulation symbol stream; c) mapping modulation symbols contained in themodulation symbol stream to at least one layer according to either oneof predetermined mapping combinations; and d) transmitting thelayer-mapped symbols, wherein each of the predetermined mappingcombinations includes a specific combination in which a single codewordconstituted by the symbol stream generated by the modulation of thespecific data block is mapped to at least two layers, and the specificcombination is designed to alternately map the symbol streamconstructing the single codeword to the at least two layers.

Preferably, the MIMO system uses a maximum of 4 layers and a maximum of2 codewords, and the predetermined mapping combination includes a firstcombination in which a single codeword is mapped to a single layer, asecond combination in which two codewords are mapped to two layers,respectively, a third combination in which two codewords are mapped tothree layers, and a fourth combination in which two codewords are mappedto four layers.

Preferably, in the third combination, a first codeword from among thetwo codewords is mapped to a first layer from among the three layers,and a second codeword from among the two codewords is mapped to secondand third layers from among the three layers.

Preferably, in the fourth combination, a first codeword from among thetwo codewords is mapped to first and second layers from among the fourlayers, and a second codeword from among the two codewords is mapped tothird and fourth layers from among the four layers.

Preferably, in the layer mapping step c), if a specific one codeword ismapped to two layers according to the third or fourth combination fromamong the predetermined mapping combinations, an even-th index symbolfrom among symbol streams constructing the specific one codeword ismapped to a first layer from among the two layers, and an odd-th indexsymbol is mapped to a second layer from among the two layers, so thatthe even-th index symbol and the odd-th index symbol are alternatelymapped to the first and second layers.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

Advantageous Effects

The method for indicating a combination between a codeword and a streamaccording to one embodiment of the present invention can rationallylimit the number of all combinations between a codeword and a stream inconsideration of a variety of aspects, so that it can reduce the numberof bits of information indicating the number of all combinations betweena codeword and a stream. As for the above-mentioned aspects, theabove-mentioned method considers a maximum transmission rate of aspecific codeword, compares the possibility of use with the number ofcases indicating a corresponding combination, maintains a combinationavailable for retransmission, improves a decoding performance of areceiver using the Successive Interference Cancellation (SIC) method,considers a stream grouping based on an antenna grouping, and considersa user's convenience in a multi-user MIMO communication system.

Therefore, the present invention indicates all codeword-streamcombinations, which are required for both an uplink and a downlink in aMIMO communication system, with less number of bits, thereby increasingthe efficiency of control information.

The present invention provides a method for mapping codeblocks accordingto layers in a MIMO communication system, transmitting the mappedcodeblocks, and additionally guaranteeing a spatial diversity gaincaused by the spatial multiplexing.

In the case where a single data block is divided into several codeblocksand the codeblocks are channel-coded, the present invention gives eachcodeblock a sufficient spatial diversity by adding simple functions to atransmission chain.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention, illustrate embodiments of the inventionand together with the description serve to explain the principle of theinvention.

In the drawings:

FIG. 1 is a block diagram illustrating a conventional MIMO communicationsystem;

FIG. 2 shows channels from N_(T) Tx antennas to an Rx antenna (i);

FIG. 3A is a conceptual diagram illustrating a single-user MIMOcommunication system;

FIG. 3B is a conceptual diagram illustrating a multi-user MIMOcommunication system;

FIG. 4 is a block diagram illustrating the relationship between acodeword and a stream in a MIMO communication system;

FIG. 5A is a block diagram illustrating a MIMO communication system inwhich a maximum number of codewords is 2 and a maximum number ofantennas is 4 according to the present invention;

FIG. 5B is a block diagram illustrating a MIMO communication system inwhich a maximum number of codewords is 2 and a maximum number ofantennas is 2 according to the present invention;

FIG. 6 is a conceptual diagram illustrating a modulation coding set(MCS) provided when a single codeword is transmitted via several streamsaccording to the present invention;

FIG. 7 is a conceptual diagram illustrating a successive interferencecancellation (SIC) concept performed in a receiver so as to improve adecoding performance of a codeword according to the present invention;

FIG. 8A shows a specific case in which a single codeword is transmittedvia two streams and it is determined whether the SIC is performed inthis case;

FIG. 8B shows a specific case in which two codewords are transmitted viatwo streams, respectively, and it is determined whether the SIC isperformed in this case;

FIG. 9 shows that two codewords are transmitted via two streams, onecodeword is successfully received and the other codeword fails to bereceived and a retransmission of the failed codeword is requested;

FIGS. 10A˜10C show a variety of methods for grouping multiple antennasin various ways;

FIG. 11 shows the number of available streams of a user in a multi-userMIMO communication system;

FIG. 12 shows a simulation result indicating a difference in systemthroughput between a first case in which only one codeword is used and asecond case in which two codewords are used on the condition that thereare several streams;

FIGS. 13A˜13C show the simulation results of a reception-end performanceaccording to the order of SIC decoding;

FIG. 14 shows the simulation result of a reception-end performance whenthe number of all combinations between a codeword and a stream islimited to a specific number of combinations capable of being indicatedby a given bit number;

FIG. 15 is a block diagram illustrating a wireless communication system;

FIG. 16 is a block diagram illustrating a transmitter according to oneembodiment of the present invention;

FIG. 17 is a block diagram illustrating a transmission according toanother embodiment of the present invention;

FIG. 18 is a block diagram illustrating a channel encoding schemeaccording to one embodiment of the present invention;

FIG. 19 is a conceptual diagram illustrating a data transmissionaccording to one embodiment of the present invention;

FIG. 20 is a conceptual diagram illustrating a data transmissionaccording to another embodiment of the present invention;

FIG. 21 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention;

FIG. 22 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention;

FIG. 23 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention;

FIG. 24A is a block diagram illustrating a data processing method of atransmitter according to an OFDMA scheme;

FIG. 24B is a block diagram illustrating a data processing method of areceiver according to an OFDMA scheme;

FIG. 25 is a conceptual diagram illustrating a method for separating asystematic part and a parity part of a coded code-block from each other,and performing a rate-matching on the separated parts;

FIGS. 26A˜26B are conceptual diagrams illustrating a single codeword(SCW) and multiple codewords (MCW), respectively;

FIG. 27 shows a coding chain used for a HS-DSCH of a WCDMA systemaccording to the present invention;

FIG. 28 shows a downlink FDD sub-frame structure of an LTE systemaccording to the present invention;

FIGS. 29A˜29B show transmission-chain structures of an LTE systemaccording to the present invention;

FIG. 30 shows a transmission-chain structure according to one embodimentof the present invention;

FIGS. 31A˜31B show transmission-chain structures according to anotherembodiment of the present invention;

FIGS. 32A˜32B show transmission-chain structures according to yetanother embodiment of the present invention; and

FIG. 33 shows a transmission-chain structure according to yet anotherembodiment of the present invention.

BEST MODE

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers will be usedthroughout the drawings to refer to the same or like parts.

Prior to describing the present invention, it should be noted that mostterms disclosed in the present invention correspond to general termswell known in the art, but some terms have been selected by theapplicant as necessary and will hereinafter be disclosed in thefollowing description of the present invention. Therefore, it ispreferable that the terms defined by the applicant be understood on thebasis of their meanings in the present invention.

For the convenience of description and better understanding of thepresent invention, general structures and devices well known in the artwill be omitted or be denoted by a block diagram or a flow chart.Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

The present invention provides a method for rationally limiting thenumber of all combinations between a codeword and a stream, and reducinga bit number of information indicating the limited number ofcombinations. For this purpose, the present invention considers themethod for rationally limiting the number of all combinations afterconsidering the number of available combinations between a codeword anda stream.

If the number of Tx/Rx antennas is limited as shown in equation 16, amaximum number of streams is limited. And then, if the number ofcodewords is limited, and a minimum number of streams is also limited.

Examples associated with the above-mentioned case will hereinafter bedescribed.

If the number of Tx/Rx antennas is 4, a maximum number of streams orcodewords is 4. Meanwhile, if the number of codewords is limited, aminimum number of available streams is limited. If the number ofcodewords is 2, the number of streams is equal to or higher than “2” ormore. Therefore, if a minimum value of Tx/Rx antennas is 4 and thenumber of codewords is 2, the number of available streams may be 2, 3,or 4. If a minimum value of Tx/Rx antennas is 2 and the number ofcodewords is 2, the number of available streams is only “2”.

Generally, the number of Tx/Rx antennas is 4 or 2 in commercial cases,and a maximum number of codewords is 2. In recent times, according tothe 3GPP LTE, a maximum number (N_(T)) of Tx antennas has been set to 4,a maximum number (N_(R)) of Rx antennas has been set to 4, and a maximumnumber of allowable multi-codewords has been set to 2, which have beenprescribed in 3GPP, R1-063013 (Approved minutes of 3GPP TSG RAN WG1 #46in Tallinn (Tallinn, Estonia, 28 Aug.˜1 Sep., 2006)), and 3GPP,R1-063613 (Approved Report of 3GPP TSG RAN WG1 #46bis (Seoul, Korea,9˜13 Oct., 2006).

Therefore, the following description of the present invention assumesthat the number of Tx/Rx antennas is 4 or 2 and the maximum number ofcodewords is 2, but the scope of the present invention is not limited tothis assumption, and can also be applied to other combinations asnecessary.

A combination method between a codeword and a stream on the conditionthat the number of Tx/Rx antennas is 4 or 2 and the maximum number ofcodewords is 2 will hereinafter be described.

FIG. 5A is a block diagram illustrating a MIMO communication system inwhich a maximum number of codewords is 2 and a maximum number ofantennas is 4 according to the present invention. FIG. 5B is a blockdiagram illustrating a MIMO communication system in which a maximumnumber of codewords is 2 and a maximum number of antennas is 2 accordingto the present invention.

Referring to FIG. 5A, if the number of antennas is “4”, a maximum numberof streams is limited to “4”. Therefore, if the number of codewords is“1”, the number of available streams is 1, 2, 3, or 4. If the number ofcodewords is “2”, the number of available streams is 2, 3, or 4.

Referring to FIG. 5B, if the number of antennas is “2”, a maximum numberof streams is limited to “2”. Therefore, if the number of codewords is“1”, the number of available streams is 1 or 2. If the number ofcodewords is “2”, the number of available streams is set to only “2”.

In this case, the present invention pays attention to the combinationbetween the codeword and the stream, instead of the combination betweenthe stream and the antenna. This combination between the codeword andthe stream is denoted by a bold-solid line in FIGS. 5A and 5B.

In fact, the combination between the stream and the antenna isdifferently decided according to categories of a MIMO system.Accordingly, a number of a stream is fixed under a given restrictioncondition, and only the combination between the codeword and the streamunder the same restriction condition will be considered.

If the maximum number of streams is 4 and the maximum number ofcodewords is 2 as shown in FIG. 5A, the following combination is made,and a detailed description thereof will hereinafter be described.

If the number of antennas is 4, i.e., if the maximum number of stream is4, and the maximum number of codewords is 2, all combinations between acodeword and a stream are as shown in the following tables 1 and 2:

[Table 1]

TABLE 1 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,{Stream1,Stream2}} 6 {Codeword,{Stream1,Stream3}}{Codeword,{Stream1,Stream4}} {Codeword,{Stream2,Stream3}}{Codeword,{Stream2,Stream4}} {Codeword,{Stream3,Stream4}} 3{Codeword,{Stream1,Stream2,Stream3}} 4{Codeword,{Stream1,Stream3,Stream4}}{Codeword,{Stream1,Stream2,Stream4}}{Codeword,{Stream2,Stream3,Stream4}} 4{Codeword,{Stream1,Stream2,Stream3,Stream4}} 1 Sub sum 15

[Table 2]

TABLE 2 C S Combination details # 2 2[{Codeword,Stream1},{codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1},{Codeword,{Stream2,Stream3}}] 12[{Codeword,Stream1},{Codeword,{Stream2,Stream4}}][{Codeword,Stream1},{Codeword,{Stream3,Stream4}}][{Codeword,Stream2},{Codeword,{Stream1,Stream3}}][{Codeword,Stream2},{Codeword,{Stream1,Stream4}}][{Codeword,Stream2},{Codeword,{Stream3,Stream4}}][{Codeword,Stream3},{Codeword,{Stream1,Stream2}}][{Codeword,Stream3},{Codeword,{Stream1,Stream4}}][{Codeword,Stream3},{Codeword,{Stream2,Stream4}}][{Codeword,Stream4},{Codeword,{Stream1,Stream2}}][{Codeword,Stream4},{Codeword,{Stream1,Stream3}}][{Codeword,Stream4},{Codeword,{Stream2,Stream3}}] 4[{Codeword,Stream1},{Codeword,{Stream2,Stream3, 4 Stream4}}][{Codeword,Stream2},{Codeword,{Stream1,Stream3, Stream4}}][{Codeword,Stream3},{Codeword,{Stream1,Stream2, Stream4}][{Codeword,Stream4},{Codeword,{Stream1,Stream2, Stream3}}][{Codeword,{Stream1,Stream2}},{Codeword,(Stream3, 3 Stream4)}][{Codeword,{Stream1,Stream3}},{Codeword,(Stream2, Stream4)}][{Codeword,{Stream1,Stream4}},{Codeword,(Stream2, Stream3)}] Sub sum 25

“C” in the table indicates the number of codewords, “S” indicates thenumber of streams, and “#” indicates the number of occurrences. Thesesymbols will be equally applied to the following tables.

In this case, Table 1 shows examples of individual combinations when thenumber of codewords is 1, and Table 2 shows examples of individualcombinations when the number of codewords is 2.

The order of codewords in the above-mentioned Table 1 or 2 is of noimportance. Generally, each codeword includes specific information(e.g., a packet number) capable of discriminating each codeword.Therefore, the present invention considers how many codewords have beentransmitted, instead of considering the order of codewords.

If each stream is mapped to an antenna, the antenna mapping operation ischanged according to the order of streams. Also, if a precoding is used,the mapping operation is also changed according to the order ofprecoding in consideration of a corresponding weight vector. Therefore,the stream has a fixed order, so that it must indicate the order ofcombinations.

As can be seen from Tables 1 and 2, if the number of codewords is 1, thenumber of combinations is 15. If the number of codewords is 2, thenumber of combinations is 25. So the total of 40 combinations arerequired. Therefore, in the case where all combinations are allowedwithout any restriction on the condition that the maximum number ofcodewords is 2 and the maximum number of streams is 4, the combinationsmust be denoted by 6 bits (2⁵=32<40<2⁶=64).

In the meantime, as shown in FIG. 5B, if the maximum number of streamsis 2, and the maximum number of codewords is 2, all the combinationsbetween the codeword and the stream are as follows.

As shown in FIG. 5B, if the number of antennas is 2, i.e., if themaximum number of streams is 2, and if the maximum number of codewordsis 2, all cases of individual combinations are as shown in the followingtable 3:

[Table 3]

TABLE 3 C S Combination details # 1 1 {Codeword,Stream1} 2{Codeword,Stream2} 2 {Codeword,{Stream1,Stream2}} 1 2 2[{Codeword,Stream1},{Codeword,Stream2}] 1 Sub sum 4

The above-mentioned Table 3 shows exemplary combinations provided whenthe number of codewords is 1 or 2.

If the maximum number of streams is 2 and the maximum number ofcodewords is 2 as shown in Table 3, it can be recognized that a total of4 codeword-stream combinations are required. In this case, if all thecombinations are allowed without any restriction, the 4 codeword-streamcombinations must be denoted by 2 bits (2¹=32<4≦2²=4). However, thiscase may be considered to be a subset of the specific case using amaximum of 4 streams shown in Table 1 or 2. Therefore, provided that thepresent invention can effectively indicate the aforementioned caseemploying the maximum of 4 streams, it can also be applied to anothercase employing a maximum of two streams.

In the meantime, in the case where the combination between a codewordand a stream is independently indicated according to the number ofantennas, this case is left out of consideration because the number ofused bits is a small number. Therefore, the following embodiments of thepresent invention will disclose a method for effectively indicating allcombinations between a codeword and a stream with less number of bits onthe basis of a specific case in which a maximum of 4 streams are used.If the combination between the codeword and the stream can be denotedwith less number of bits, a transmission efficiency of a control signalcan be improved.

If a given limitation between a codeword and a stream is that themaximum number of streams is 4 and the maximum number of codewords is 2,all the combinations between the codeword and the stream are madeavailable, so that a total of 6 bits are required to indicate all thecombinations without any restriction. One embodiment of the presentinvention provides a method for limiting all combinations between thecodeword and the stream to reduce the number of information unitsindicating a used combination from among the combinations.

In order to implement the above-mentioned embodiment, a method forlimiting the number of Tx streams via which each codeword istransmitted, and reducing the number of all combinations between thecodeword and the stream will hereinafter be described.

FIG. 6 is a conceptual diagram illustrating a modulation coding set(MCS) provided when a single codeword is transmitted via several streamsaccording to the present invention.

Referring to FIG. 6, if several streams are transmitted via multipleantennas, each stream experiences a variety of channel environments. Inthis case, if a single codeword (e.g., a codeword 1) is transmitted viaseveral streams (e.g., streams 1˜3), different channel environments ofthe individual streams are immediately averaged during the decoding of acorresponding codeword.

As shown in FIG. 6, if the codeword 1 is transmitted via a first stream1 based on a 256QAM and an MCS of a coding rate=8/9, a second stream 2based on a 64QAM and an MCS of a coding rate=1/2, and a third stream 3based on a BPSK and an MCS of a coding rate=1/10, the codeword 1 has thesame effect as in the case in which the codeword 1 is transmitted withan average MCS level of MCS levels of the first to third streams, sothat the efficiency may be lower than that of another case in which acodeword requiring high-speed transmission is transmitted via a goodstream.

In the light of channel capacity, it is most preferable that eachcodeword be adaptively transmitted according to channel environments ofindividual streams, so that it is also preferable that a single codewordbe transmitted to each stream. In other words, provided there are fourcodewords if four streams are used to transmit the four codewords, theoptimum condition can be provided.

However, according to the above-mentioned case in which a maximum of 4streams are used and a maximum of 2 codewords are used, if a maximum of4 streams must be used under a given condition, each codeword must betransmitted via at least two streams.

Therefore, one embodiment of the present invention limits the number ofTx streams via which each codeword is transmitted, so that a givencodeword is transmitted via a minimum number of streams from among allstreams. As a result, the number of all combinations between a codewordand a stream is reduced.

In more detail, according to this embodiment in which a maximum of 4streams are used and a maximum of 2 codewords are used, it is preferableto limit the codeword-stream combinations such that a single codewordshould be limited up to two streams.

Then, the combination between the codeword and the stream can berepresented by the following tables 4 and 5:

[Table 4]

TABLE 4 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,{Stream1,Stream2}} 6 {Codeword,{Stream1,Stream3}}{Codeword,{Stream1,Stream4}} {Codeword,{Stream2,Stream3}}{Codeword,{Stream2,Stream4}} {Codeword,{Stream3,Stream4}} Sub sum 10

[Table 5]

TABLE 5 C S Combination details # 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1},{Codeword,(Stream2, 12 Stream3)}][{Codeword,Stream1},{Codeword,(Stream2, Stream4)}][{Codeword,Stream1},{Codeword,(Stream3, Stream4)}][{Codeword,Stream2},{Codeword,(Stream1, Stream3)}][{Codeword,Stream2},{Codeword,(Stream1, Stream4)}][{Codeword,Stream2},{Codeword,(Stream3, Stream4)}][{Codeword,Stream3},{Codeword,(Stream1, Stream2)}][{Codeword,Stream3},{Codeword,(Stream1, Stream4)}][{Codeword,Stream3},{Codeword,(Stream2, Stream4)}][{Codeword,Stream4},{Codeword,(Stream1, Stream2)}][{Codeword,Stream4},{Codeword,(Stream1, Stream3)}][{Codeword,Stream4},{Codeword,(Stream2, Stream3)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 21

The above-mentioned Table 4 shows an exemplary combination between acodeword and a stream when the number of codewords is 1. Theabove-mentioned table 5 shows an exemplary combination between acodeword and a stream when the number of codewords is 2.

As can be seen from Tables 4 and 5, provided that a single codeword istransmitted via a maximum of 2 streams according to one embodiment ofthe present invention, the number of combinations is 10 when a maximumnumber of codewords is 1, and the number of combinations is 21 when amaximum number of codewords is 2, so that a total of 31 combinations arerequired. The 31 combinations can be denoted by 5 bits (2⁴=16<31<2⁵=32).

In the meantime, a more preferred embodiment of the present inventionlimits the number of streams used for transmitting a single codeword,and at the same time removes a specific combination having many morenumbers of cases than the possibility to be used, so that the number ofall combinations can be further reduced, and a detailed descriptionthereof will hereinafter be described.

In more detail, one embodiment of the present invention further assumesa specific condition which excludes a specific case in which threestreams are adapted to transmit the codeword from all combinations shownin Tables 4 and 5.

Although the above-mentioned case in which three streams are usedcorresponds to a channel rank of 3, the channel rank of 3 may be changedto another rank, so that the changed rank may be indicated. The reasonwhy the aforementioned case in which three streams from among severalstreams are used is excluded is that the number of combinations made inthe case of three streams is 12 as shown in Table 5 so that 12combinations occupy about 38.7% of all combinations, but the possibilityof selecting the case in which three streams are used is about 20%according to the simulation result.

Therefore, it is preferable that the case having many more combinationsthan the possibility to be selected for transmission be excluded fromall the cases. As a result, the above-mentioned embodiment of thepresent invention removes the combination having many numbers of casesfrom all the combinations between a codeword and a stream, so that thenumber of bits of control information is effectively reduced.

A detailed example associated with the above-mentioned description canbe represented by the following Table 6:

[Table 6]

TABLE 6 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,Stream1,Stream2}} 6 {Codeword,Stream1,Stream3}}{Codeword,Stream1,Stream4}} {Codeword,Stream2,Stream3}}{Codeword,Stream2,Stream4}} {Codeword,Stream3,Stream4}} 2 2[{Codeword,Stream1,{Codeword,Stream2}] 6[{Codeword,Stream1,{Codeword,Stream3}][{Codeword,Stream1,{Codeword,Stream4}][{Codeword,Stream2,{Codeword,Stream3}][{Codeword,Stream2,{Codeword,Stream4}][{Codeword,Stream3,{Codeword,Stream4}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, Stream2,Stream3)}] Sub sum 19

According to the embodiment of the present invention shown in Table 6,provided that a specific combination having many more numbers of cases(i.e., the case in which three streams are used) than the possibility ofuse is removed, the number of combinations between a codeword and astream is 10 when a maximum number of codewords is 1, and the number ofcombinations between a codeword and a stream is 9 when a maximum numberof codewords is 2, so that a total of 19 combinations are required.

Therefore, an embodiment in which a single codeword is transmitted via amaximum of 2 streams and the case employing three streams is excludedcan be denoted by 5 bits (2⁴=16<19<2⁵=32). In this case, the number ofbits of control information indicating all combinations may be inferiorto that of Table 4 or 5, however, if the above-mentioned case is appliedto another example, all combinations can be denoted with less number ofbits.

In the meantime, another embodiment of the present invention limits thenumber of streams used for transmitting a single codeword, and at thesame time removes a specific combination having many more numbers ofcases than the possibility of to be used. And, if two streams are usedto improve a reception performance, the above-mentioned embodiment ofthe present invention removes a combination in which a single codewordis transmitted via the two streams, so that it can further reduce thenumber of all combinations between a codeword and a stream. A detaileddescription of the above-mentioned embodiment will hereinafter bedescribed.

FIG. 7 is a conceptual diagram illustrating a successive interferencecancellation (SIC) concept performed in a receiver so as to improve adecoding performance of a codeword according to the present invention.

If several codewords are received in a receiver, the receiver mayperform the SIC to improve a decoding performance of a Rx signal. If thereceiver simultaneously receives a codeword 1 and a codeword 2 as shownin FIG. 7, it firstly decodes the codeword 1, removes all signalsassociated with the decoded codeword 1 from all Rx signals, and decodesthe codeword 2 using the remaining signals, so that a decodingperformance of all the signals can be improved.

In this case, in order to perform the above-mentioned SIC, there is aneed for several codewords to be transmitted as shown in FIG. 7. If asingle codeword is distributed to several streams and be thentransmitted although several streams are received, the receiver isunable to perform the SIC shown in FIG. 7.

FIG. 8A shows a specific case in which a single codeword is transmittedvia two streams and it is determined whether the SIC is performed inthis case. FIG. 8B shows a specific case in which two codewords aretransmitted via two streams, respectively, and it is determined whetherthe SIC is performed in this case.

Referring to FIG. 8A, if a single codeword is transmitted via twostreams, an independent codeword is not contained in each of two Rxstreams, so that the SIC cannot be applied to the case of FIG. 8A.

Referring to FIG. 8B, if two codewords are transmitted via two streams,the SIC shown in FIG. 7 can be applied to a Rx signal transmitted viaeach stream, so that the decoding performance of the Rx signal can beimproved.

Therefore, another embodiment of the present invention limits the numberof streams used for transmitting a single codeword as shown in Table 6(e.g., 2 streams or below 2), and removes a specific combination havingmany more numbers of cases than the possibility of use (e.g., thecombination in which three streams are allowed is removed). And, if twostreams are used to further limit the number of bits indicating allcodeword-stream combinations, the above-mentioned embodiment of thepresent invention removes a combination (shown in FIG. 8A) in which asingle codeword is transmitted via two streams, so that a receiver canimprove a decoding performance of a Rx signal using the SIC.

In this case, the above-mentioned example has no combination in whichthe number of streams is 3. If the number of streams is 1, 2 or 4, onlyone codeword-stream combination exists as shown in the following table7:

[Table 7]

TABLE 7 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2 2[{Codeword,Stream1,{Codeword,Stream2}] 6[{Codeword,Stream1,{Codeword,Stream3}][{Codeword,Stream1,{Codeword,Stream4}][{Codeword,Stream2,{Codeword,Stream3}][{Codeword,Stream2,{Codeword,Stream4}][{Codeword,Stream3,{Codeword,Stream4}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, Stream2,Stream3)}] Sub sum 13

With reference to Table 7, the number of combinations is 4 when amaximum number of codewords is 1, and the number of combinations is 9when a maximum number of codewords is 2, so that a total of 13combinations are required. Therefore, in the case of this exampleaccording to this embodiment, all combinations between a codeword and astream can be denoted by 4 bits (2³=8<13<2⁴=16). Therefore, in the lightof the number of bits indicating all the combinations, the method ofTable 7 is better than the method of Table 6.

In the meantime, the above-mentioned embodiments of the presentinvention have considered a channel capacity, an efficiency ofcombination indication, and an Rx performance improvement based on theSIC in order to effectively reduce the number of all combinationsbetween a codeword and a stream.

In the case of considering only the efficiency in reduction of thenumber of all codeword-stream combinations, if a receiver of an HARQcommunication system generates a retransmission request, an unexpectedproblem may occur in properly dealing with the retransmission request.

Therefore, another embodiment of the present invention limits the numberof all codeword-stream combinations by removing a specific combinationhaving many more numbers of cases than the possibility of to be usedfrom all the combinations, and maintains a combination available for aretransmission scheme such as a HARQ. A detailed description of theabove-mentioned embodiment will hereinafter be described.

FIG. 9 shows that two codewords are transmitted via two streams, onecodeword is successfully received and the other codeword fails to bereceived and a retransmission of the failed codeword is requested.

Referring to FIG. 9, if two codewords are transmitted via two streamsduring a first transmission, it is assumed that a codeword 1 between thetwo codewords is successfully received, and a codeword 2 correspondingto the other codeword fails to be received so that a NACK signal istransmitted to a transmitter. In this case, if the “Chase Combining”scheme is applied to retransmission, it is preferable that the codeword2 be retransmitted via two streams.

However, the above-mentioned combination is not contained in thecombinations shown in Table 7. Therefore, one embodiment of the presentinvention removes a specific combination having many more numbers ofcases than the possibility to be used from all the combinations (e.g., acombination with three streams), and maintains the above-mentionedcombination available for the retransmission scheme such as the HARQ.The above-mentioned embodiment of the present invention can berepresented by the following tables 8 and 9:

[Table 8]

TABLE 8 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 6 {Codeword,(Stream1,Stream3)}{Codeword,(Stream1,Stream4)} {Codeword,(Stream2,Stream3)}{Codeword,(Stream2,Stream4)} {Codeword,(Stream3,Stream4)} 4{Codeword,(Stream1,Stream2,Stream3,Stream4)} 1 Sub sum 11

[Table 9]

TABLE 9 C S Combination details # 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 9

The above-mentioned Table 8 shows a combination between a codeword and astream when a single codeword is used. The above-mentioned Table 9 showsa combination between a codeword and a stream when two codewords areused.

As can be seen from Tables 8 and 9, a specific combination having manymore numbers of cases than the possibility of use (i.e., a specific casewith three streams) is excluded from all combinations of theabove-mentioned example in which a maximum of 4 streams are used and amaximum of 2 codewords are used. If a specific combination in which asingle codeword capable of being used for retransmission is transmittedvia two or four streams, and a combination in which each of twocodewords are transmitted via one or two streams are maintained, 11combinations are made available for the case in which a single codewordis used, and 9 combinations are made available for the other case inwhich two codewords are used, so that a total of 20 combinations aremade available. Therefore, all the combinations can be denoted by bits(2⁴=16<20<2⁵=32).

5 bits are required as a bit number for indicating the combinationbetween a codeword and a stream shown in Tables 8 and 9, so that thenumber of combinations for use in a specific case can be furtherlimited, resulting in reduction of the number of correspondingcombinations. The following table 10 shows that a combination in whichtwo codewords are transmitted via two streams is further limited.

[Table 10]

TABLE 10 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 6 {Codeword,(Stream1,Stream3)}{Codeword,(Stream1,Stream4)} {Codeword,(Stream2,Stream3)}{Codeword,(Stream2,Stream4)} {Codeword,(Stream3,Stream4)} 4{Codeword,(Stream1,Stream2,Stream3,Stream4)} 1 2 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 14

With reference to Table 10, the number of combinations is 11 when amaximum number of codewords is 1, and the number of combinations is 3when a maximum number of codewords is 2, so that a total of 14combinations are required. Therefore, the above-mentioned case in whichthe case of three streams is considered is maintained, and two codewordsare transmitted via two streams can be denoted by control informationcomposed of 4 bits (2³=8<13<2⁴=16).

In the meantime, according to detailed embodiments of the presentinvention concerning Tables 3˜10, the number of bits (i.e., a bitnumber) of control information required for indicating all combinationsbetween a codeword and a stream is determined to be a bit number capableof indicating the power of 2 higher than the number of all combinations.In this case, the number of codeword-stream combinations capable ofbeing denoted by a bit number of required control information isgenerally higher than the number of all combinations to which apredetermined restriction is applied.

Therefore, in association with Tables 3˜10, another embodiment of thepresent invention adds additional combinations corresponding to adifference (e.g., 3 combinations in Table 10) between the number ofcombinations (e.g., 16 combinations in Table 10), denoted by the numberof information pieces (e.g., 4 bits in Table 10) indicating all thelimited combinations, and the number of all the limited combinations(e.g., 13 combinations in Table 10) to all the limited combinations, anda detailed description thereof will hereinafter be described.

The cases shown in Tables 6˜10 indicate the cases in which thecombination for the case that the number of cases is higher than thepossibility to be used, such as the case when the number of streams is“3”, is excluded, so that the number of bits required for indicating allcombinations can be greatly reduced. However, the above-mentioned casesshown in Tables 6˜10 are unable to indicate the case in which threestreams are used whereas they are able to reduce the number ofcombinations, resulting in the occurrence of an unexpected problem.

In the meantime, there is a difference between the maximum number ofcombinations denoted by the number of bits required for indicating allcombinations and the number of really-used combinations. In other words,it is assumed that the number of real combinations is M and N bits arerequired for indicating the combinations according to an equation of2^(N-1)<M≦2^(N). As a result, a predetermined number of combinationscorresponding to a difference (2^(N)−M) between the number (2^(N)) ofmaximum-expressible combinations denoted by N bits and the number (M) ofreal combinations can be further added.

Therefore, one embodiment of the present invention adds specificcombinations in which three streams are used. The number of specificcombinations corresponds to a difference between the number ofcombinations, denoted by a bit number required for indicating allcombinations, and the number of all real combinations. So, thisembodiment provides a method for using three streams without increasingthe number of bits required for indicating all combinations.

In more detail, Table 7 may indicate all combinations (i.e., 13combinations) limited by 4-bit control information. In this case, aspecific combination employing three streams corresponding to adifference between 16 combinations capable of being denoted by controlinformation and 13 combinations may be added to the combinations ofTable 7. In this way, the case of Table 10 may further include twocombinations, each of which uses three streams.

However, the number of combinations occupied by the case employing threestreams is higher than the number of combinations capable of beingadded. In other words, the number of cases, each of which uses a singlecodeword, from among several cases each of which uses two streams, is 4,and the number of other cases, each of which uses two codewords is 12.Therefore, the combination addition is selectively performed on some ofthe above-mentioned cases.

For example, three combinations can be added to Table 7, so that it isassumed that only successive numbers are selected from amongthree-stream combinations. In other words, it is assumed that the orderof “Stream1, Stream2, and Stream3”, “Stream2, Stream3, and Stream4”, or“Stream3, Stream4, and Stream1” is selected. In fact, the stream numbersare fixed in ascending numerical order, so that the order “Stream3,Stream4, and Stream1” may be considered to be “Stream1, Stream3, andStream4” for the convenience of description.

If a combination employing a single codeword is added, the addedcombination may be {Codeword, (Stream1, Stream2, Stream3)}, {Codeword,(Stream2, Stream3, Stream4)} or {Codeword, (Stream1, Stream3, Stream4)},as represented by the following table 11:

[Table 11]

TABLE 11 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 3{Codeword,(Stream1,Stream2,Stream3)} 3{Codeword,(Stream2,Stream3,Stream4)}{Codeword,(Stream1,Stream3,Stream4)} 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 14

In Table 11, the added combination is denoted by a shaded part.

And, if a combination employing two codewords is added, the addedcombination may be [{Codeword, Stream1}, {Codeword, (Stream2,Stream3)}], [{Codeword, Stream2}, {Codeword, (Stream3, Stream4)}], and[{Codeword, Stream1}, {Codeword, (Stream3, Stream4)}], as represented bythe following table 12:

[Table 12]

TABLE 12 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1),(Codeword,(Stream2, 3 Stream3)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}][{Codeword,Stream1),(Codeword,(Stream3, Stream4)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 14

In Table 12, the added combinations are denoted by shaded parts.

In the meantime, for another example, two combinations can be added toTable 10, so that it is assumed that only successive numbers areselected from among three-stream combinations. In other words, it isassumed that the orders of “Stream1, Stream2, and Stream3”, and“Stream2, Stream3, and Stream4” are selected. In the case of adding aspecific case of a combination employing a single codeword, {Codeword,(Stream1, Stream2, Stream3)}, and {Codeword, (Stream2, Stream3,Stream4)} may be added. In the case of adding another case of acombination employing two codewords, [{Codeword, Stream1}, {Codeword,(Stream2, Stream3)}] and [{Codeword, Stream2}, {Codeword, (Stream3,Stream4)}] may be added. The above-mentioned combinations can be shownin the following Tables 13 and 14:

[Table 13]

TABLE 13 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 6 {Codeword,(Stream1,Stream3)}{Codeword,(Stream1,Stream4)} {Codeword,(Stream2,Stream3)}{Codeword,(Stream2,Stream4)} {Codeword,(Stream3,Stream4)} 3{Codeword,(Stream1,Stream2,Stream3)} 3{Codeword,(Stream2,Stream3,Stream4)}{Codeword,(Stream1,Stream3,Stream4)} 4{Codeword,(Stream1,Stream2,Stream3,Stream4)} 1 2 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 16

[Table 14]

TABLE 14 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 6 {Codeword,(Stream1,Stream3)}{Codeword,(Stream1,Stream4)} {Codeword,(Stream2,Stream3)}{Codeword,(Stream2,Stream4)} {Codeword,(Stream3,Stream4)} 4{Codeword,(Stream1,Stream2,Stream3,Stream4)} 1 2 3[{Codeword,Stream1),(Codeword,(Stream2, 2 Stream3)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 3 (Stream3,Stream4)}][{Codeword,(Stream1,Stream3)},{Codeword, (Stream2,Stream4)}][{Codeword,(Stream1,Stream4)},{Codeword, (Stream2,Stream3)}] Sub sum 16

In Tables 13 and 14, the added combinations are denoted by shaded parts.

In the meantime, another embodiment of the present invention considers agrouping of streams to be a rational method for limiting the number ofcombinations between a codeword and a stream, and a detailed descriptionthereof will hereinafter be described.

If four antennas are used for the MIMO technology, the individualantennas must configure unique channel environments independent of eachother in order to implement an optimum status. For this purpose, theindividual antennas are physically spaced apart from each other.

However, the streams are grouped by various reasons when the antennasare actually used, so that antennas contained in each group maycorrelate with each other.

FIGS. 10A˜10C show a variety of methods for grouping multiple antennasin various ways.

FIG. 10A shows an ideal case in which four antennas configureindependent channels without any grouping. However, four antennas arespaced apart from each other by a predetermined distance, so that atransmitter or a receiver may have insufficient space to configureindependent channels. Specifically, it is difficult for the structure ofFIG. 10A to be applied to a mobile terminal.

In order to solve the above-mentioned problem, the structure of FIG. 10Bhas been widely used. In this structure of FIG. 10B, two antennas aregrouped, antennas of each group have related channels, and otherantennas of different groups have independent channels. And, anotherstructure of FIG. 10C may also be used to solve the above-mentionedproblem. In this structure of FIG. 10C, two antennas are grouped by across polarized diversity (CPD) scheme, antennas of one group arehorizontally polarized, and antennas of the other group are verticallypolarized.

In another antenna-grouping example, if a total of 4 antennas are used,channel conditions of two antennas are stably measured, and channelconditions of the remaining two antennas are unstable so that an errorfrequently occurs in the remaining two antennas. Under this situation,antennas having a stable channel condition are grouped, and the otherantennas having an unstable channel condition are grouped, so that thefour antennas are grouped two by two.

As described above, if the antennas are grouped and limited, streamscorresponding to the antennas are also limited, and it can be recognizedthat predetermined streams are grouped.

Therefore, one embodiment of the present invention provides a method forlimiting the number of all combinations between a codeword and a stream.In more detail, the above-mentioned embodiment limits the number ofstreams, each of which transmits a single codeword, as shown in Tables 4and 5, and groups streams. And, if a single codeword is transmitted viastreams corresponding to the number of streams contained in a singlegroup, the above-mentioned embodiment controls the single codeword to betransmitted via the streams contained in a single group, so that thenumber of combinations between a codeword and a stream is reduced. Adetailed description of the above-mentioned embodiment will hereinafterbe described.

In a detailed example, it is assumed that a first stream 1 and a secondstream 2 form a single group, and a third stream 3 and a fourth stream 4form another group. In this case, according to one embodiment of thepresent invention, if a single codeword is transmitted via two streams,this codeword may be allocated to only a single group, and all thelimited combinations are shown in the following Table 15:

[Table 15]

TABLE 15 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 2 {Codeword,(Stream3,Stream4)} 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1),(Codeword,(Stream3, 4 Stream4)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}][{Codeword,Stream3),(Codeword,(Stream1, Stream2)}][{Codeword,Stream4),(Codeword,(Stream1, Stream2)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 1 (Stream3,Stream4)}] Sub sum17

With reference to Table 15, if a maximum number of codewords is 1, thenumber of available combinations is 6. If a maximum number of codewordsis 2, the number of available combinations is 11, so that a total of 17combinations are made available.

Therefore, as described above, if a single codeword is transmitted via amaximum of 2 streams, several streams are grouped two by two, and thesingle codeword is transmitted via two streams, the present inventionallows the single codeword to be transmitted via only streams containedin the same group. In this case, all codeword-stream combinations can bedenoted by a maximum of 5 bits (2⁴=16<17<2⁵=32).

In the meantime, according to another embodiment of the presentinvention, if the combination between a codeword and a stream is limitedand at the same time several codewords are transmitted in associationwith Table 15, the above-mentioned embodiment allows the codewords to betransmitted via streams of different groups, or allows the codewords tobe transmitted via streams of the same group, so that the number of allcodeword-stream combinations can be further reduced. A detaileddescription of the above-mentioned embodiment will hereinafter bedescribed.

In association with Table 15, if several codewords are transmitted tofurther reduce the number of bits required for indicating allcombinations, i.e., if two codewords are transmitted, individualcodewords are transmitted via streams of different groups or otherstreams of the same group, so that they are transmitted via differentchannel environments, resulting in acquisition of a diversity gain.

In this case, the above-mentioned case in which two codewords aretransmitted via streams of different group may indicate that streamshaving the same or similar channel environments in their grouping aregrouped into a single group. The other case in which two codewords aretransmitted via streams of the same group may indicate that streamshaving different channel environments in their grouping are grouped intoa single group. In other words, if two codewords are transmitted viadifferent channel environments, they are transmitted via streams ofdifferent groups or other streams of the same group according to astream grouping method.

In association with Table 15, if a first stream 1 and a second stream 2configure a single group, and a third stream 3 and a fourth stream 4configure another single group, the above-mentioned case in which twocodewords are transmitted via streams of different groups can be denotedby the following Table 16:

[Table 16]

TABLE 16 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 2 {Codeword,(Stream3,Stream4)} 2 2[{Codeword,Stream1},{Codeword,Stream3}] 4[{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}] 3[{Codeword,Stream1),(Codeword,(Stream3, 4 Stream4)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}][{Codeword,Stream3),(Codeword,(Stream1, Stream2)}][{Codeword,Stream4),(Codeword,(Stream1, Stream2)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 1 (Stream3,Stream4)}] Sub sum15

In Table 16, the limited parts are denoted by shaded parts.

With reference to Table 16, if a maximum number of codewords is 1, thenumber of all combinations is 6. If a maximum number of codewords is 2,the number of all combinations is 9, so that a total of 15 combinationsare made available. Therefore, in the above-mentioned example of thepresent invention, a maximum of 4 bits (2³=8<15<2⁴=16) are required toindicate all combinations between a codeword and a stream.

In the meantime, the above-mentioned case in which two codewords aretransmitted via streams of the same group can be denoted by thefollowing Table 17:

[Table 17]

TABLE 17 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2{Codeword,(Stream1,Stream2)} 2 {Codeword,(Stream3,Stream4)} 2 2[{Codeword,Stream1},{Codeword,Stream3}] 2[{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1),(Codeword,(Stream3, 4 Stream4)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}][{Codeword,Stream3),(Codeword,(Stream1, Stream2)}][{Codeword,Stream4),(Codeword,(Stream1, Stream2)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 1 (Stream3,Stream4)}] Sub sum13

In Table 17, the limited parts are denoted by shaded parts.

With reference to Table 17, if a maximum number of codewords is 1, thenumber of all combinations is 6. If a maximum number of codewords is 2,the number of all combinations is 7, so that a total of 13 combinationsare made available. Therefore, in the above-mentioned example of thepresent invention, a maximum of 4 bits (2³=8<15<2⁴=16) are required toindicate all combinations between a codeword and a stream.

In the meantime, another embodiment of the present invention provides amethod for limiting the number of codewords or streams in associationwith Tables 4 and 5. In more detail, the above-mentioned embodimentlimits the number of Tx streams via which a single codeword istransmitted, groups streams using Tables 15˜17, allows at least twostreams to be transmitted via at least two codewords, and allows areceiver to perform the SIC scheme related with FIG. 7, so that itreduces the number of all codeword-stream combinations and improves adecoding performance of a Rx signal.

In other words, in association with FIGS. 7, 8A, and 8B, if severalstreams (e.g., two streams) are transmitted, and the individual streamshave independent codewords, a Rx signal receiving the independentcodewords may improve a decoding performance of each codeword. However,if a single codeword is distributed to the individual streams and bethen transmitted, a receiver is unable to perform the SIC scheme.

Therefore, provided that at least two streams transmit at least twocodewords, the following Table 18 is made:

[Table 18]

TABLE 18 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] 3[{Codeword,Stream1),(Codeword,(Stream3, 4 Stream4)}][{Codeword,Stream2),(Codeword,(Stream3, Stream4)}][{Codeword,Stream3),(Codeword,(Stream1, Stream2)}][{Codeword,Stream4),(Codeword,(Stream1, Stream2)}] 4[{Codeword,(Stream1,Stream2)},{Codeword, 1 (Stream3,Stream4)}] Sub sum15

With reference to Table 18, if a maximum number of codewords is 1, thenumber of all codeword-stream combinations is 4. If a maximum number ofcodewords is 2, the number of all codeword-stream combinations is 11, sothat a total of 15 combinations are made available. Therefore, in theabove-mentioned example of the present invention, a maximum of 4 bits(2³=8<15<2⁴=16) of a control signal are required to indicate allcombinations between a codeword and a stream.

The above-mentioned embodiments of the present invention assume that asingle-user MIMO communication system is used. However, in the case ofconsidering a multi-user MIMO communication system, a condition betweena codeword and a stream may be changed to another condition.

Another embodiment of the present invention provides a method forrationally limiting the number of all codeword-stream combinations inconsideration of the above-mentioned multi-user environment, and adetailed description thereof will hereinafter be described.

In order to acquire a multi-user diversity gain from the multi-user MIMOcommunication system, a searching process for users having channelsorthogonal to each other is of importance. If a single user uses allstreams, a single-user MIMO system is made so that the user cannotacquire a multi-user diversity gain. In order to implement themulti-user MIMO communication system, it is preferable that the numberof streams capable of being used by each user be reduced.

This embodiment of the present invention limits the number of allcodeword-stream combinations to effectively acquire a multi-userdiversity gain from the single-user or multi-user MIMO communicationsystem. For the convenience of description, it is assumed that a maximumnumber of streams is 4 and a maximum number of codewords is 2 in thefollowing examples.

Preferably, the multi-user MIMO communication system may select usershaving channels orthogonal to each other from among several users.However, in fact, there is a low probability of selecting only theorthogonal users. The multi-user MIMO communication system has a highprobability of selecting users having channels orthogonal to each other,so that it can be more stably operated.

FIG. 11 shows the number of available streams of a user in a multi-userMIMO communication system.

Referring to FIG. 11, if a single user uses at least two streams,another limitation is applied to a user's selection process.

In other words, if a single user uses two or more streams, channelspassing through all streams of a corresponding user must be orthogonalto each other. The user who uses two or more streams must be selectedfrom among several users satisfying the following condition. Thereafter,a process for searching for specific users having channels orthogonal toeach other from among corresponding users must be performed. In thiscase, a variety of methods may be used as a final selection, forexample, a Max Sum-Rate method.

In more detail, as shown in FIG. 11, if a first user 1 uses a firststream 1 and a second stream 2, two channels of the first user 1employing the first and second streams 1 and 2 must be orthogonal toeach other, and the first user 1 must be selected from among users whosatisfy the above-mentioned orthogonal condition.

A channel of a second user 2 employing a third stream 3 must beorthogonal to a channel of a third user employing a fourth stream 4, theprocess for selecting users who satisfy the above-mentioned orthogonalcondition is more difficult than that of another case in which all usersuse a single stream. Therefore, in order to support the above-mentionedcase in which a single user uses several streams, the above-mentionedoperation can be easily performed under the condition that many moreusers exist.

Therefore, in order to easily select desired users in the multi-userMIMO communication system, one embodiment of the present inventionallows a single user to use only one stream to limit the number of allcodeword-stream combinations. In this case, available combinationsbetween a codeword and a stream can be represented by the followingTable 19:

[Table 19]

TABLE 19 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} Sub sum 4

As can be seen from Table 19, in order to easily select a desired userin the multi-user MIMO communication system, the above-mentionedembodiment of the present invention limits the number of codeword-streamcombinations so that a single user can use only one stream. In thiscase, the number of all combinations is 4, and can be denoted by amaximum of 2 bits (2¹<4<2²=4).

In the meantime, in association with Table 19, the above-mentionedembodiment of the present invention allows a single user to use only onestream, so that the multi-user MIMO communication system can easilyselect an orthogonal channel between users, however, it should be notedthat a maximum peak-rate of each user may be damaged.

Therefore, in order to solve the above-mentioned problem, anotherembodiment of the present invention allows a single user to use severalstreams, but it allows the individual streams to use differentcodewords. A detailed description of the above-mentioned embodiment willhereinafter be described.

In more detail, provided that a single user uses two streams, a maximumpeak-rate of each user can be higher than that of the case shown inTable 19. Also, the above-mentioned embodiment of the present inventionallows the individual streams to use different codewords, and appliesthe SIC scheme to a Rx signal received in a receiver, so that it canimprove a decoding performance.

Under this situation, all combinations between a codeword and a streamcan be represented by the following Table 20:

[Table 20]

TABLE 20 C S Combination details # 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] Sub sum 6

With reference to Table 20, the number of all codeword-streamcombinations is 6, and can be denoted by a maximum of 3 bits(2²=4<6<2³=8).

In the meantime, in association with Tables 19 and 20, theabove-mentioned embodiments of the present invention have theaforementioned advantages and disadvantages. A multi-user MIMOcommunication system according to another embodiment of the presentinvention accommodates a codeword-stream combination to which theadvantages and disadvantages are applied, and may selectively use thecombination according to various situations.

In this case, all combinations between a codeword and a stream can berepresented by the following Table 21:

[Table 21]

TABLE 21 C S Combination details # 1 1 {Codeword,Stream1} 4{Codeword,Stream2} {Codeword,Stream3} {Codeword,Stream4} 2 2[{Codeword,Stream1},{Codeword,Stream2}] 6[{Codeword,Stream1},{Codeword,Stream3}][{Codeword,Stream1},{Codeword,Stream4}][{Codeword,Stream2},{Codeword,Stream3}][{Codeword,Stream2},{Codeword,Stream4}][{Codeword,Stream3},{Codeword,Stream4}] Sub sum 10

In this case, as can be seen from Table 21, if a single user uses asingle stream, 4 combinations are made available. If a single user usestwo streams, 6 combinations are made available, so that a total of 10combinations are made available. Therefore, the above-mentionedcombinations can be denoted by a maximum of 4 bits (2³=8<10<2⁴=16).

The above-mentioned embodiments have disclosed a logical method forreducing the number of combinations between a codeword and a stream.

In the meantime, one embodiment of the present invention will disclose amethod for additionally reducing the number of codeword-streamcombinations in consideration of an Rx-scheme and Rx-performance of areceiver, and a detailed description thereof will hereinafter bedescribed.

If an SIC-based receiver is used as a receiver of the present inventionand several codewords exist, an overall system performance may bechanged according to the protection order of the codewords. Therefore,the order of codewords in each combination must be considered.

Firstly, as shown in FIG. 5A, if a maximum number of streams is 4 and amaximum number of codewords is 2, the following combinations areprovided.

If the number of antennas is 4, i.e., if a maximum number of streams is4, a maximum number of codewords is 2. In this case, all availablecodeword-stream combinations are shown in the following Tables 22 and23:

[Table 22]

TABLE 22 C S Combination details # 1 1 {Codeword1,Stream1} 4{Codeword1,Stream2} {Codeword1,Stream3} {Codeword1,Stream4} 2{Codeword1,(Stream1,Stream2)} 6 {Codeword1,(Stream1,Stream3)}{Codeword1,(Stream1,Stream4)} {Codeword1,(Stream2,Stream3)}{Codeword1,(Stream2,Stream4)} {Codeword1,(Stream3,Stream4)} 3{Codeword1,(Stream1,Stream2,Stream3)} 4{Codeword1,(Stream1,Stream2,Stream4)}{Codeword1,(Stream1,Stream2,Stream4)}{Codeword1,(Stream2,Stream3,Stream4)} 4{Codeword1,(Stream1,Stream2,Stream3,Stream4)} 1 Sub sum 15

[Table 23]

TABLE 23 C S Combination details # 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 12[{Codeword1,Stream2},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream1}][{Codeword1,Stream2},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream2}][{Codeword1,Stream3},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream3}] 3[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 24[{Codeword1,(Stream2,Stream3)},{Codeword2,Stream1},][{Codeword1,Stream1},{Codeword2,(Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,(Stream3,Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,Stream1}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream3)},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,(Stream3,Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,Stream2}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream2)},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,(Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,Stream3}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream2)},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream3)},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,(Stream2,Stream3)}][{Codeword1,(Stream2,Stream3)},{Codeword2,Stream4}] 4[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3, 8 Stream4)}][{Codeword1,(Stream2,Stream3,Stream4)},{Codeword2, Stream1}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3, Stream4)}][{Codeword1,(Stream1,Stream3,Stream4)},{Codeword2, Stream2}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2, Stream4)}][{Codeword1,(Stream1,Stream2,Stream4)},{Codeword2, Stream3}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2, Stream3)}][{Codeword1,(Stream1,Stream2,Stream3)},{Codeword2, Stream4}][{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 6 Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream3)},{Codeword2, (Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream4)},{Codeword2, (Stream2,Stream3)}][{Codeword1,(Stream2,Stream3)},{Codeword2,(Stream1, Stream4)}] Sub sum50

Table 22 shows examples of individual combinations when the number ofcodewords is 1. Table 23 shows examples of individual combinations whenthe number of codewords is 2.

The order of codewords in Table 22 or 23 is denoted by successivenumbers. The codeword number indicates a decoding order when a receiveruses the SIC-based receiver. Thereafter, the codeword 1 is firstlydecoded as shown in FIG. 7, an interference signal related with thecodeword 1 is removed from the Rx signal, and then a codeword 2 isdecoded.

In other words, the antenna mapping operation is changed according tothe order of streams when each stream is re-mapped to the antenna. Also,if a pre-coding method is used, the antenna mapping operation is changedaccording to the pre-coding order in association with a correspondingweight vector. Therefore, the streams have a fixed order, so that theorder of the streams in a combination must also be indicated.

As can be seen from Tables 22 and 23, if the number of codewords is 1,the number of combinations is 15. If the number of codeword is 2, thenumber of combination is 50, so that a total of 65 combinations arerequired. Therefore, in order to allow all combinations without anyrestriction under the condition that a maximum number of codewords is 2and a maximum number of streams is 4, a maximum of 7 bits(2⁶=64<65<2⁷=128) are needed.

In the meantime, as shown in FIG. 5B, all codeword-stream combinationsprovided when a maximum number of streams is 2 and a maximum number ofcodewords is 2 are as follows.

As shown in FIG. 5B, if the number of antennas is 2, i.e., if a maximumnumber of streams is 2, a maximum number of codewords is 2. All cases ofindividual combinations are shown in the following Table 24:

[Table 24]

TABLE 24 C S Combination details # 1 1 {Codeword1,Stream1} 2{Codeword1,Stream2} 2 {Codeword1,(Stream1,Stream2)} 1 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 2[{Codeword1,Stream2},{Codeword2,Stream1}] Sub sum 5

Table 24 shows all examples of individual combinations provided when thenumber of codewords is 1 or 2.

As shown in Table 14, if a maximum number of streams is 2 and a maximumnumber of codewords is 2, a total of 5 codeword-stream combinations areneeded. In this case, in order to allow all combinations without anyrestriction under the condition that the maximum number of streams is 2and the maximum number of codewords is 2, a maximum of 3 bits(2²=4<5<2³=8) are needed. However, the above-mentioned case may beconsidered to be a subset of the case in which a maximum of 4 streamsshown in Tables 22 and 23 are used. Therefore, if the above-mentionedcase in which a maximum of 4 streams are used can be effectivelyindicated, it should be noted that this case can also be applied toanother case in which a maximum of 2 streams are used.

As described above, the above-mentioned case in which the maximum numberof streams requires a small number of bits to independently indicate thecombination between a codeword and a stream, so that it is left out ofconsideration. Therefore, the following embodiment of the presentinvention will disclose a method for effectively indicating allcodeword-stream combinations with less number of bits on the basis ofthe above-mentioned case in which a maximum of 4 streams are used. Ifthe combination between a codeword and a stream can be denoted with lessnumber of bits, Tx efficiency of a control signal can be enhanced.

As previously stated above, if a given restriction condition between acodeword and a stream is that a maximum number of streams is 4 and amaximum number of codewords is 2, 65 combinations between the codewordand the stream are made available. In order to indicate all the 65combinations without any restriction, a total of 7 bits are needed.

One embodiment of the present invention will disclose a method forlimiting all combinations between a codeword and a stream inconsideration of an Rx-scheme and Rx-performance of a receiver, so thatit can reduce the number of information pieces indicating usedcombinations from among all combinations.

FIG. 12 shows a simulation result indicating a difference in systemthroughput between a first case in which only one codeword is used and asecond case in which two codewords are used on the condition that thereare several streams.

In FIG. 12, “SCW” is an abbreviation of a Single Codeword, “MCW” is anabbreviation of Multiple Codewords. “MMSE” indicates a specific case inwhich a minimum mean square error (MMSE)-based receiver is used in areceiver. “MMSE+SIC” indicates that a specific receiver capable ofperforming interference cancellation by applying the SIC scheme to theMMSE-resultant signal is used in a receiver. “Ior/Ioc” on a horizontalaxis indicates the ratio of a Tx power of a Node-B to an interferencepower. The ratio of the Tx power to the interference power can becalculated by a SINR, and is physically similar to the SINR.

A detailed simulation assumption of FIG. 12 is as follows. Two Txantennas and two Rx antennas are used, a MCS scheme based on the 3GPP TR25.892 is used, and the simplest Per Antenna Rate Control (PARC) schemeis used as the MIMO scheme. It is assumed that “Pedestrian B” modelproposed by the ITU is used as a simulation channel, and a mobileterminal has a speed of 3 km/h. And, it is assumed that an OFDM schemeis used as a transmission scheme, the length of FFT is 1024, the numberof sub-carriers actually used for a bandwidth of 10 MHz is 600, and thesize of a cyclic prefix (CP) is 74.

As can be seen from the result of FIG. 12, in the case of transmittingtwo streams, a first case in which the two streams are distributed totwo codewords has a good performance superior to that of a second casein which the two streams are transmitted via a single codeword. Theabove-mentioned result has the same result in the light of the number ofstreams or codewords. If the above-mentioned result is generalized, thefollowing result may be acquired. In other words, if several streams areused, it is preferable that several codewords instead of a singlecodeword be used to increase an overall system performance. Therefore,if several streams are used to limit the number of all combinationsbetween a codeword and a stream, one embodiment of the present inventionprovides a method for limiting a combination which allows a singlecodeword to use all streams.

In more detail, as described above, the above-mentioned embodiment, inwhich a maximum of 4 streams are used and a maximum of 2 codewords areused, may allow desired data to be transmitted via two codewords whenthe codeword-stream combination has two or more streams.

The combination between a codeword and a stream according to theabove-mentioned embodiment of the present invention can be representedby the following Table 25:

[Table 25]

TABLE 25 C S Combination details # 1 1 {Codeword1,Stream1} 4{Codeword1,Stream2} {Codeword1,Stream3} {Codeword1,Stream4} 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 12[{Codeword1,Stream2},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream1}][{Codeword1,Stream2},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream2}][{Codeword1,Stream3},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,Stream3}] 3[{codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 24[{Codeword1,(Stream2,Stream3)},{codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,(Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,Stream1}][{Codeword1,Stream1},{Codeword2,(Stream3,Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,Stream1}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream3)},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,Stream2}][{Codeword1,Stream2},{Codeword2,(Stream3,Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,Stream2}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream2)},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,Stream3}][{Codeword1,Stream3},{Codeword2,(Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,Stream3}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream2)},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream3)},{Codeword2,Stream4}][{Codeword1,Stream4},{Codeword2,(Stream2,Stream3)}][{Codeword1,(Stream2,Stream3)},{Codeword2,Stream4}] 4[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3, 8 Stream4)}][{Codeword1,(Stream2,Stream3,Stream4)},{Codeword2, Stream1}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3, Stream4)}][{Codeword1,(Stream1,Stream3,Stream4)},{Codeword2, Stream2}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2, Stream4)}][{Codeword1,(Stream1,Stream2,Stream4)},{Codeword2, Stream3}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2, Stream3)}][{Codeword1,(Stream1,Stream2,Stream3)},{Codeword2, Stream4}][{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 6 Stream4)}][{Codeword1,(Stream3,Stream4)},{Codeword2,(Stream1,Stream2)}][{Codeword1,(Stream1,Stream3)},{Codeword2, (Stream2,Stream4)}][{Codeword1,(Stream2,Stream4)},{Codeword2,(Stream1,Stream3)}][{Codeword1,(Stream1,Stream4)},{Codeword2, (Stream2,Stream3)}][{Codeword1,(Stream2,Stream3)},{Codeword2,(Stream1, Stream4)}] Sub sum54

With reference to Table 25, provided that several codewords can beavailable when several streams are used, and there are restrictions forallowing several codewords to be used instead of a single codeword, thenumber of combinations is 4 when a maximum number of codewords is 1, thenumber of combinations is 50 when a maximum number of codewords is 2, sothat a total of 54 combinations are needed. These 54 combinations can bedenoted by a maximum of 6 bits (2⁵=32<54<2⁶=64).

A more preferred embodiment of the present invention provides a methodfor fixing the decoding order based on the SIC scheme to reduce thenumber of all combinations.

FIGS. 13A˜13C show the simulation results of a reception-end performanceaccording to the order of SIC decoding.

It can be recognized that all combinations shown in Table 25 include aspecific part indicating the decoding order when the SIC scheme is usedin a receiver. In order to indicate the decoding order of codewords andassociated SIC receiver, the combination between a codeword and a streamis symmetrically repeated.

For example, if two streams (i.e., stream 1 and stream 2) are used,first combinations (Codeword1, Stream1) and (Codeword2, Stream2) exist,and second combinations (Codeword1, Stream2) and (Codeword2, Stream1)also exist. The decoding orders of the first combinations aresymmetrically different from those of the second combinations. Thesimulation result indicating a performance difference between the firstand second combinations is shown in FIG. 13A. FIG. 13B shows a case inwhich three streams are used, and FIG. 13C shows another case in whichfour streams are used. The simultaneously environments of FIGS. 13A˜13Care different in the number of antennas, and the remaining parts ofFIGS. 13A˜13C are equal to those of FIG. 12. In more detail, FIG. 13Ashows two antennas, FIG. 13B shows three antennas, and FIG. 13C showsfour antennas, and the remaining parts other than the number of antennasare equal to those of FIG. 12.

In order to analyze the simulation result, the case of FIG. 13Cillustrating a performance of the case employing 4 streams willhereinafter be described.

In FIG. 13A, “Max” is a specific case in which all original combinationsindicating the SIC-decoding order are included. “(12, 34) L1, L2 first”is a specific case, in which streams 1 and 2 (or layers 1 and 2) L1 andL2 are assigned to a first codeword 1, the streams 1 and 2 of the firstcodeword 1 are firstly decoded during the SIC decoding when streams 3and 4 (or layers 3 and 4) are used for a second codeword 2, and then thestreams 3 and 4 of a second codeword 2 are decoded. In this way, “(34,12) L3, L4 first” is a specific case, in which streams 3 and 4 areassigned to a first codeword 1, streams 1 and 2 are assigned to a secondcodeword 2, the streams 3 and 4 corresponding to the first codeword 1are firstly decoded, and then the other streams 1 and 2 corresponding tothe second codeword 2 are decoded.

As can be seen from the result of FIG. 13C, in the case of (12,34) or(34,12) in which two streams are assigned to each codeword, there isalmost no difference in performance between the case of (12,34) and theother case of (34,12).

Also, in the case of (1,234) or (234,1) in which a single stream isassigned to a single codeword and three streams are assigned to otherstreams, there is almost no difference in order between the case of(1,234) and the case of (234,1).

In another aspect, the case in which two streams are assigned to eachcodeword has a good performance superior to that of the other case inwhich a single stream is assigned to a single codeword and three streamsare assigned to other streams. Although the above-mentioned case inwhich two streams are assigned to each codeword has a performanceslightly lower than that of the other case in which all cases areallowed, it should be noted that their performances of theabove-mentioned two cases are very close to each other. Therefore, inthe performance aspect, it is preferable that the SIC-decoding order befixed to only a predetermined order, and a maximum of 2 codewords beselected by a single codeword.

FIG. 13B shows different performances when three streams are used, inwhich a single stream is assigned to a single codeword and two streamsare assigned to the other codeword. In this case, it is more preferablethat the codeword composed of the two streams be decoded after thesingle stream has been decoded, resulting in the implementation of ahigher performance.

FIG. 13A shows a performance when a total of 2 streams are used, inwhich a single stream is assigned to each codeword. In this case, it canbe recognized that there is almost no difference in performance betweendecoded codewords, irrespective of the decoding order of the codewords.

The following three results are acquired by the performances of FIGS.13A˜13C.

According to a first result, the decoding order of the SIC receiver isfixed, so that the codeword 1 is firstly decoded, and the codeword 2 isthen decoded.

According to a second result, if the number of streams assigned to thefirst codeword 1 is asymmetrical to the number of streams assigned tothe second codeword 2, a codeword with less number of streams from amongthe first and second codewords 1 and 2 is firstly decoded. For example,if the number of all combinations is 3, one codeword has a single streamand the other codeword has two streams, the codeword composed of onlyone stream is firstly decoded. For this purpose, a single stream isassigned to the first codeword 1, and two streams are assigned to thesecond codeword 2.

According to a third result, a maximum of 2 streams are assigned to asingle codeword.

In more detail, according to this embodiment in which a maximum of 4streams and a maximum of 2 codewords are used, it is assumed that thedecoding order of the SIC receiver in all combinations shown in Tables22 and 23 is fixed, the allocation process having a symmetricalstructure to indicate the SIC decoding order is removed, and the SICreceiver firstly decodes the first codeword 1 and then decodes thesecond codeword 2. Namely, the term “SIC decoding order” used in theabove-mentioned embodiment of the present invention is indicative of thedecoding order of individual codewords when the SIC receiver is used.

Also, if a total of 3 streams exist, the above-mentioned embodimentconsiders only a specific case, in which a single stream is assigned toa first codeword 1 and two streams are assigned to a second codeword 2.And, if a total of 4 streams exist, the above-mentioned embodimentconsiders only a specific case, in which two streams are assigned to afirst codeword 1 and the remaining two streams are assigned to a secondcodeword 2.

A detailed example associated with the above-mentioned embodiment isshown in the following Table 26:

[Table 26]

TABLE 26 C S Combination details # 1 1 {Codeword1,Stream1} 4{Codeword1,Stream2} {Codeword1,Stream3} {Codeword1,Stream4} 2{Codeword1(Stream1,Stream2)} 6 {Codeword1,(Stream1,Stream3)}{Codeword1,(Stream1,Stream4)} {Codeword1,(Stream2,Stream3)}{Codeword1,(Stream2,Stream4)} {Codeword1,(Stream3,Stream4)} 3{Codeword1,(Stream1,Stream2,Stream3)} 4{Codeword1,(Stream1,Stream2,Stream4)}{Codeword1,(Stream1,Stream2,Stream4)}{Codeword1,(Stream2,Stream3,Stream4)} 4{Codeword1,(Stream1,Stream2,Stream3,Stream4)} 1 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 6[{Codeword1,Stream1},{Codeword2,Stream3}][{Codeword1,Stream1},{Codeword2,Stream4}][{Codeword1,Stream2},{Codeword2,Stream3}][{Codeword1,Stream2},{Codeword2,Stream4}][{Codeword1,Stream3},{Codeword2,Stream4}] 3[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 12[{Codeword1,Stream1},{Codeword2,(Stream2,Stream4)}][{Codeword1,Stream1},{Codeword2,(Stream3,Stream4)}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3)}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream4)}][{Codeword1,Stream2},{Codeword2,(Stream3,Stream4)}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2)}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream4)}][{Codeword1,Stream3},{Codeword2,(Stream2,Stream4)}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2)}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream3)}][{Codeword1,Stream4},{Codeword2,(Stream2,Stream3)}] 4[{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 3 Stream4)}][{Codeword1,(Stream1,Stream3)},{Codeword2,(Stream2, Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,(Stream2, Stream3)}] Sub sum36

Therefore, a detailed embodiment of the present invention has a total of36 cases, which are denoted by a maximum of 6 bits (2⁶=32<36<2⁶=64). Ineach case, the SIC-decoding order is fixed, a single codeword has amaximum of 2 streams, and a codeword with less number of streams isfirstly decoded in the case of an asymmetrical stream. In the light ofthe number of bits of control information indicating all combinations,the above-mentioned case has a gain of 1 bit as compared to the casesshown in Tables 22 and 23.

In the meantime, the method for reducing the number of cases in theabove-mentioned combination may also be applied to the above-mentionedembodiment. In other words, if several streams are used, severalcodewords are used, a single codeword has a maximum of 2 streams, and acodeword with less number of streams is fixed to a first codeword, sothat the number of all combinations can be limited.

Provided that the SIC-based receiver is used in a receiver, severalcodewords are used when several streams are used, and a single codewordhas a maximum of 2 streams. In the case of an asymmetrical stream, acodeword with less number of streams is firstly decoded, and theSIC-decoding order is fixed, so that the number of all combinations canbe limited.

A detailed example associated with the above-mentioned description isshown in the following Table 27:

[Table 27]

TABLE 27 C S Combination details # 1 1 {Codeword1,Stream1} 4{Codeword1,Stream2} {Codeword1,Stream3} {Codeword1,Stream4} 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 6[{Codeword1,Stream1},{Codeword2,Stream3}][{Codeword1,Stream1},{Codeword2,Stream4}][{Codeword1,Stream2},{Codeword2,Stream3}][{Codeword1,Stream2},{Codeword2,Stream4}][{Codeword1,Stream3},{Codeword2,Stream4}] 3[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 12[{Codeword1,Stream1},{Codeword2,(Stream2,Stream4)}][{Codeword1,Stream1},{Codeword2,(Stream3,Stream4)}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream3)}][{Codeword1,Stream2},{Codeword2,(Stream1,Stream4)}][{Codeword1,Stream2},{Codeword2,(Stream3,Stream4)}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream2)}][{Codeword1,Stream3},{Codeword2,(Stream1,Stream4)}][{Codeword1,Stream3},{Codeword2,(Stream2,Stream4)}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream2)}][{Codeword1,Stream4},{Codeword2,(Stream1,Stream3)}][{Codeword1,Stream4},{Codeword2,(Stream2,Stream3)}] 4[{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 3 Stream4)}][{Codeword1,(Stream1,Stream3)},(Codeword2,(Stream2, Stream4)}][{Codeword1,(Stream1,Stream4)},{Codeword2,(Stream2, Stream3)}] Sub sum25

Therefore, a detailed embodiment of the present invention has a total of25 cases. In each case, several codewords are used when several streamsare used, and a single codeword has a maximum of 2 streams. And, in thecase of an asymmetrical stream, i.e., if individual codewords usedifferent numbers of streams, a codeword with less number of streams isassigned to a first codeword.

Also, a detailed description embodiment of the present invention has atotal of 25 cases. In each case, if several streams are used, severalcodewords are used in consideration of usages of the SIC decodercontained in a receiver, the SIC-decoding order is fixed, and a singlecodeword has a maximum of 2 streams. And, in the case of an asymmetricalstream, i.e., if individual codewords use different numbers of streams,a codeword with less number of streams is firstly decoded. Therefore,the number of cases is 25, so that it must be denoted by a maximum of 5bits (2⁴=16<25<2⁵=32). In this case, in the light of the number of bitsof control information indicating all combinations, the above-mentionedcase has a gain of 2 bits as compared to the cases shown in Tables 22and 23.

As described above, each stream has a plurality of combinations. But,the present invention aims to reduce the number of combinations for eachstream.

FIG. 14 shows the simulation result of a reception-end performance whenthe number of all combinations between a codeword and a stream islimited to a specific number of combinations capable of being indicatedby a given bit number.

In more detail, FIG. 14 shows the simulation result in which 4 Txantennas and 4 Rx antennas were used, i.e., a maximum number of streamsis 4. Other simulation assumptions other than the number of antennas areequal to those of FIG. 12.

A detailed description of the simulation result of FIG. 14 willhereinafter be described. In the case where the number of allcombinations is determined to be Tables 22 and 23, this case is denotedby “max”. If only one combination is allowed for each stream, the numberof all combinations is 4, 4 combinations are denoted by 2 bits, asrepresented by “bit2”. If two combinations are allowed for each stream,8 cases are denoted by 3 bits, as represented by “bit3”. If 4combinations are allowed for each stream, 16 cases are denoted by 4bits, as represented by “bit4”.

As can be seen from the simulation result of FIG. 14, if only onecombination is allowed for each stream, this case has a performancealmost similar to that of another case in which all combinations aremade available.

As can be seen from the simulation result of FIG. 14, although acombination selected for reducing combinations as many as the number ofcorresponding bits is arbitrarily selected, there is almost nodifference in performance between this case and the above-mentioned casein which all combinations are made available. Therefore, in the aspectof reducing the number of cases in each combination, it is mostpreferable that the number of cases in each combinations be reduced to 4combinations.

In a detailed example associated with the above-mentioned description,if a maximum of 4 streams and a maximum of 2 codewords are used, the “2bit” case capable of being denoted by a minimum of combinations can berepresented by the following Table 28:

[Table 28]

TABLE 28 C S Combination details # 1 1 {Codeword1,Stream1} 1 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 1 3[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 1 4[{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 1 Stream4)}] Sub sum4

Therefore, a detailed embodiment of the present invention has a total of4 cases. In each case, if several streams are used, several codewordsare used, a single codeword has a maximum of 2 streams, a codeword withless number of streams is set to a first codeword in the case of anasymmetrical stream, each stream uses only one combination.

Also, a detailed embodiment of the present invention has a total of 4cases. In each case, if several streams are used in consideration of aspecific case in which a receiver uses the SIC decoder, severalcodewords are used, the SIC decoding order is fixed, a single codewordhas a maximum of 2 streams, a codeword with less number of streams isfirstly decoded in the case of an asymmetrical stream, and only onestream is allowed for each stream. The total number of cases is 4, sothat 4 cases must be denoted by a maximum of 2 bits (2¹=2<4≦2²=4). Inthis case, in the light of the number of bits of control informationindicating all combinations, the above-mentioned case has a gain of 5bits as compared to the cases shown in Tables 22 and 23.

In the meantime, the “3 bit” case from among other detailed examplesmust select two combinations for each stream, and the “4 bit” case mustselect four combinations for each stream. The above-mentioned examplesare well known to those skilled in the art, so that their detaileddescription will be omitted for the convenience of description. In thiscase, a process for selecting a combination corresponding to a fixednumber may be arbitrarily executed. By the simulation result, althoughthe combination is arbitrarily selected, there is almost no differencein performance.

In the meantime, considering a specific case in which a HARQ (HybridARQ) scheme is used in the above-mentioned combination, the addition ofa combination may also be considered. If the number of codewords is 2,one codeword has an error, and the other codeword has no error, thenumber of retransmitted codewords is only one. If a chase combiningtechnique which has been widely used for the HARQ is used for theretransmission, two codewords should be retransmitted without anymodification from the first transmission. Therefore, the retransmittedcodewords must be transmitted via one or two streams. A detaileddescription thereof will hereinafter be described.

It is assumed that two codewords are transmitted via three streamsduring a first transmission. Also, it is assumed that one of the twocodewords has an error and the other one has no error. Especially, theretransmitted codeword is assumed to be only one first codeword, so thatit is transmitted via only one stream. The case in which a singlecodeword is transmitted via a single stream is contained in an originalcombination, so that this case can be made available.

However, for another example, if a first codeword has no error and asecond codeword has an error, a retransmitted codeword is only onesecond codeword and must be transmitted via two streams.

In addition, the case in which one codeword is transmitted via twostreams is not contained in the original combination. In order toretransmit a desired codeword of the first transmission without anychange, a combination in which a single codeword is mapped to twostreams may also be added as necessary. The above-mentioned case isshown in the following Table 29:

[Table 29]

TABLE 29 C S Combination details # 1 1 {Codeword1,Stream1} 1 2[{Codeword1,Stream1,Stream2}] 1 2 2[{Codeword1,Stream1},{Codeword2,Stream2}] 1 3[{Codeword1,Stream1},{Codeword2,(Stream2,Stream3)}] 1 4[{Codeword1,(Stream1,Stream2)},{Codeword2,(Stream3, 1 Stream4)}] Sub sum5

The above-mentioned embodiments of the present invention have discloseda variety of methods for minimizing the number of codeword-streamcombinations from among all available combinations according to thenumber of given codewords and streams (or layers). A method for allowingthe combination to effectively process data under a given condition, andtransmitting a signal will hereinafter be described.

As described above, a general wireless communication system performs achannel coding to reliably transmit data. This channel coding indicatesthat a transmission performs a coding on Tx information using a forwarderror correction code, so that a channel error can be corrected by areceiver. The receiver demodulates an Rx signal, decodes the forwarderror correction code, and recovers Tx information. In this decodingprocess, the Rx-signal error caused by the channel is corrected.

An example of the forward error correction code is a turbo-code. Theturbo-code includes at least two recursive systematic convolutionencoders and an interleaver connecting between the at least tworecursive systematic convolution encoders. The larger the data block,the higher the performance of the turbo-code. An actual communicationsystem divides a predetermined-sized data block into severalsmaller-sized data blocks, and performs encoding on the divided blocks,so that it is really convenient to implement the actual communicationsystem. These smaller-sized data blocks are called codeblocks. Theforward error correction coding process is performed in units of apredetermined-sized codeblock, is mapped to wireless resources, and isthen transmitted to a destination.

If wireless resources are mapped after the channel encoding is performedin units of a codeblock by the MIMO communication system, the spatialmultiplexing is needed. The individual MIMO channels are independent ofeach other. If the spatial multiplexing of codeblocks is performed formultiple Tx antennas, a transmission efficiency can be improved.

Therefore, a method for effectively transmitting channel-encoded datavia multiple Tx antennas is needed.

Therefore, one embodiment of the present invention provides a method foreffectively transmitting data in consideration of the spatialmultiplexing by the MIMO communication system, and a detaileddescription thereof will hereinafter be described.

A wireless communication system may be based on an Orthogonal FrequencyDivision Multiplexing (OFDM) scheme. This OFDM scheme uses severalorthogonal sub-carriers. The OFDM scheme uses orthogonality between anInverse Fast Fourier Transform (IFFT) and a Fast Fourier Transform(FFT). The transmitter performs the IFFT on data, and transmits theIFFT-resultant data. The transmitter uses the IFFT to combinemulti-subcarriers. In order to separate the multi-subcarriers from eachother, the receiver uses the FFT corresponding to the multi-subcarrier.The OFDM scheme reduces the complexity of the receiver under frequencyselective fading environments of a broadband channel, uses differentchannel characteristics of sub-carriers, and performs selectivescheduling in a frequency domain, thereby increasing spectralefficiency. The Orthogonal Frequency Division Multiple Access (OFDMA)scheme is a multiple access scheme based on the OFDM scheme. By theOFDMA scheme, different sub-carriers are assigned to multiple users, sothat the efficiency of wireless resources increases.

FIG. 15 is a block diagram illustrating a wireless communication system.

The wireless communication system has been widely used to provide avariety of communication services, for example, voice or packet data.

Referring to FIG. 15, the wireless communication system includes a userequipment (UE) 10 and a base station (BS) 20. The user equipment (UE) 10may be fixed or have mobility. The user equipment (UE) may also becalled a Mobile Station (MS), a User Terminal (UT), a Subscriber Station(SS) or a wireless device. The base station (BS) 20 may be a fixedstation communicating with the user equipment (UE) 10, or may also becalled a Node-B, a Base Transceiver System (BTS), or an Access Point(AP). A single base station (BS) 20 may have one or more cells.

The term “downlink” is indicative of a communication path from the basestation (BS) 20 to the user equipment (UE) 20. The term “uplink” isindicative of a communication path from the user equipment (UE) 10 tothe base station (BS) 20. A transmitter for use in the downlink may besome parts of the base station (BS) 20, or a receiver may be some partsof the user equipment (UE) 10. A transmitter for use in the uplink maybe some parts of the user equipment (UE) 10, or a receiver may be someparts of the base station (BS) 20.

FIG. 16 is a block diagram illustrating a transmitter according to oneembodiment of the present invention.

Referring to FIG. 16, a transmitter 100 includes a CRC attachment unit110, a codeblock segmentation unit 115, a channel encoder 120, aninterleaver 130, a rate matching unit 140, a mapper 150, a layer mapper160-, and a precoding unit 170. The transmitter 100 includes Nt Txantennas (190-1, . . . , 190-Nt) (where Nt>1).

The CRC attachment unit 110 attaches a cyclic redundancy check (CRC)code for detecting an error to input data. The codeblock segmentationunit 155 segments the CRC-added code into codeblock units. In this case,the CRC code may be attached to data, and be then segmented intocodeblock units. Otherwise, the CRC code may be attached to data incodeblock units.

The channel encoder 120 performs channel encoding on codeblocks. Theinterleaver 130 performs interleaving on the channel-encoded codes. Therate matching unit 140 adjusts the interleaved codes according to theamount of wireless resources used for actual transmission. The ratematching may be conducted by a puncturing or repetition process. Themapper 150 maps the rate-matched code to a symbol indicating a signalconstellation location. An interleaver (not shown) may be located beforethe mapper 150. Namely, the interleaver may be located between the ratematching unit 140 and the mapper 150.

The layer mapper 160 performs mapping of input symbols according toindividual layers caused by the spatial multiplexing. Themapped-resultant data for each layer is called a datastream. Theprecoding unit 170 precodes an input datastream according to the MIMOscheme based on transmission antennas (190-1, . . . 190-Nt).

In this case, several datastreams are generated from a singlechannel-encoded code in the system of FIG. 16, so that this system iscalled a single codeword (SCW) system.

FIG. 17 is a block diagram illustrating a transmission according toanother embodiment of the present invention.

Compared with the transmitter 100 shown in FIG. 16, the transmission ofFIG. 17 generates several datastreams upon receiving at least onechannel-encoded code, so that the system of FIG. 17 is called a multiplecodeword (MCW) system.

Referring to FIG. 17, the transmitter 200 includes a plurality of CRCattachment units (210-1, . . . , 210-K) (where K>1), a plurality ofcodeblock segmentation units (215-1, . . . , 215-K), a plurality ofchannel encoders (220-1, . . . , 220-K), a plurality of interleavers(230-1, . . . , 230-K), a plurality of rate matching units (240-1, . . ., 240-K), a plurality of mappers (250-1, . . . , 250-K), a layer mapper260, and a precoding unit 270. The transmitter 200 includes Nt Txantennas (290-1, . . . , 290-Nt) (where Nt>1).

The CRC attachment unit (210-1, . . . , 210-K) attaches a cyclicredundancy check (CRC) code for detecting an error to input data. Thecodeblock segmentation unit (215-1, . . . , 215-K) segments theCRC-added code into codeblock units. The channel encoder (220-1, . . . ,220-K) performs channel encoding on codeblocks. The interleaver (240-1,. . . , 230-K) performs interleaving on the channel-encoded codes. Therate matching unit (240-1, . . . , 240-K) adjusts the interleaved codesaccording to the amount of wireless resources used for actualtransmission. The rate matching may be conducted by a puncturing orrepetition process. The mapper (250-1, . . . , 250-K) maps therate-matched code to a symbol indicating a signal constellationlocation. An interleaver (not shown) may be located between the ratematching unit (240-1, . . . , 240-K) and the mapper (250-1, . . . ,250-K).

The layer mapper 260 performs mapping of input symbols according toindividual layers caused by the spatial multiplexing. Themapped-resultant data for each layer is called a datastream. Thisdatastream may also be called a layer. The precoding unit 270 precodesan input datastream according to the MIMO scheme based on transmissionantennas (290-1, . . . , 290-Nt).

FIG. 18 is a block diagram illustrating a channel encoding schemeaccording to one embodiment of the present invention.

Referring to FIG. 18, the channel coding, the interleaving, and the ratematching are performed on a single codeblock, so that the singlecodeblock is transmitted via several datastreams. The codeblock is apredetermined-sized data block for performing the channel encoding. Thecodeblock may have the same size, and several codeblocks may havedifferent sizes.

Referring to FIG. 18, the channel encoder 320 performs the channelencoding on an input codeblock. The channel encoder 320 may be aturbo-code. The turbo-code may include a recursive systematicconvolution encoder and an interleaver. The turbo-code generates asystematic bit and a parity bit in bit units upon receiving the inputcodeblock. In this case, it is assumed that a code rate is ⅓, and asingle systematic block S and two parity blocks P1 and P2 are generated.The systematic block is a set of systematic bits, and the parity blockis a set of parity bits.

The interleaver 330 performs interleaving on the channel-encodedcodeblock, so that it reduces the influence of a burst error caused byRF-channel transmitter. The interleaver 330 may perform the interleavingon the systematic block S and each parity block P1 or P2, respectively.

The rate matching unit 340 adjusts the channel-encoded codeblockaccording to the size of radio resources. The rate matching may beperformed in units of the channel-encoded codeblock. Or, the systematicblock S and two parity blocks P1 and P2 are separated from each other,so that the rate matching may be performed on each of them.

A data transmitter method based on the spatial multiplexing willhereinafter be described.

For the convenience of description and better understanding of thepresent invention, it is assumed that data is transmitted via twodatastreams (i.e., two layers).

The systematic block S and two parity blocks P1 and P2 generated from asingle codeblock are equally distributed to two datastreams, and arethen transmitted to a destination. In the case where the codeblock isequally distributed to two data blocks and be then transmitted to thedestination, a system may acquire the spatial diversity, resulting inthe increased performance. The systematic block S is more important forthe decoding as compared to two parity blocks P1 and P2. So, if thesystematic block S is transmitted to a datastream having a betterchannel condition, the performance may be improved. In this case, if twodatastreams are mapped to radio resources, a specific pattern may beused as necessary.

Next, it is assumed that there are two or more codeblocks to betransmitted. In this case, it is assumed that three codeblocks areequally distributed to two datastreams, and are then transmitted to adestination.

FIG. 19 is a conceptual diagram illustrating a data transmissionaccording to one embodiment of the present invention.

Referring to FIG. 19, two datastreams (i.e., two layers) are distributedto a frequency domain during a single subframe, so that they areseparated from each other in the frequency domain. A single subframe isindicative of a frequency domain, which includes a plurality of resourceblocks. A single resource block includes a plurality of sub-carriers.For example, a single resource block may include 12 sub-carriers. Asingle subframe is indicative of a time domain including two slots, eachof which includes 7 OFDM symbols. However, the above-mentioneddescription exemplarily defines the number of resource blocks, thenumber of slots, and the number of OFDM symbols contained in the singlesubframe, so that the scope of the present invention is not limited tothe above-mentioned values, and can also be applied to other examples.

A single codeblock is equally mapped to two datastreams. Radio resourcesallocated to the single codeblock are equally allocated to the twodatastreams. After the single codeblock has been mapped, the nextcodeblock is mapped by the same method as in the mapped codeblock. Inthis case, three codeblocks are equally distributed to two datastreams,and are then mapped. In this case, an interval occupied in the timedomain may be a minimum interval.

In the case of using the turbo-code, a single codeblock is divided intoa systematic block S and two parity blocks P1 and P2. The systematicblock S and the parity blocks P1 and P2 are equally distributed to twodatastreams, and are then transmitted to a destination. The mapping ofthe systematic block S and the parity blocks P1 and P2 may have aspecific pattern. Specifically, the systematic block S is more importantfor the error correction as compared to the parity blocks P1 and P2, sothat the systematic block S may be equally distributed to twodatastreams, and be then transmitted to a destination. Therefore, thespatial diversity gain for the systematic block S may be acquired, orthe systematic block S may be mapped to a datastream having a goodchannel condition.

The codeblock is distributed to two datastreams, and is then mapped.And, two datastreams are transmitted via multiple antennas, so thespatial diversity gain caused by the datastream is acquired. Thecodeblock is equally mapped to two datastreams, so that the decodingdelay caused by transmission of the datastreams can be reduced.

When the codeblock is mapped to N datastreams (where N>1 and N=evennumber), it can be equally mapped to the N datastreams. If N is an oddnumber, the codeblock may be maximum-equally mapped to the Ndatastreams.

FIG. 20 is a conceptual diagram illustrating a data transmissionaccording to another embodiment of the present invention.

In more detail, FIG. 20 shows an exemplary case in which a codeblock isdistributed to two datastreams, and is then transmitted via the twodatastreams.

Referring to FIG. 20, a first codeblock is mapped to one of the twodatastreams, and a second codeblock is mapped to the other datastream. Athird codeblock is mapped via two datastreams.

When a single codeblock is mapped to a single datastream, a redundantcodeblock may occur. In other words, when M codeblocks (where M>1) aremapped to N datastreams (where N>1), the relationship between the M andN values is not denoted by a multiple, as denoted by M=k×N+q (k=integer,0<q<N−1). In this case, the q codeblock may be distributed to Ndatastreams, and be then mapped to them.

If a single codeblock includes a systematic block S and parity blocks P1and P2, the systematic block S and the parity blocks P1 and P2 may bemapped to a single datastream according to a specific pattern.

FIG. 21 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention.

Referring to FIG. 21, a first codeblock and a second codeblock aremapped to two datastreams according to a specific pattern. The firstcodeblock and the second codeblock cross each other in units of an OFDMsymbol. A third codeblock is mapped via two datastreams.

If M codeblocks are transmitted during L OFDM symbol intervals, thefirst codeblock is mapped to N datastreams during a “ceil(L/M)”interval, and the second codeblock is mapped. “ceil(x)” may be a minimuminteger higher than “x”. Symbols from a first OFDM symbol to theceil(L/M)−1 OFDM symbol are fully filled with data, but the ceil(L/M)-thOFDM symbol may be partially filled. Subsequently, the next codeblock ismapped.

If a single codeblock includes the systematic block S and parity blocksP1 and P2, the systematic block S and the parity blocks P1 and P2 may beequally mapped with two datastreams. The mapping of the systematic blockS and the parity blocks P1 and P2 via the two datastreams may have aspecific pattern.

FIG. 22 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention.

Referring to FIG. 22, a first codeblock and a second codeblock aremapped to two datastreams according to a specific pattern. The firstcodeblock and the second codeblock cross each other in units of aresource block. A third codeblock is mapped via two datastreams.

If a single codeblock includes the systematic block S and parity blocksP1 and P2, the systematic block S and the parity blocks P1 and P2 may beequally mapped with two datastreams. The mapping of the systematic blockS and the parity blocks P1 and P2 via the two datastreams may have aspecific pattern.

FIG. 23 is a conceptual diagram illustrating a data transmissionaccording to yet another embodiment of the present invention.

Referring to FIG. 23, three codeblocks are mapped via all subframes. Thethree codeblocks are transmitted via two datastreams. The threecodeblocks may be mapped to two datastreams according to a specificpattern.

In this case, 3 codeblocks are mapped one by one in units of a resourceblock (i.e., on a frequency axis), and the 3 codeblocks may be mappedone by one in units of an OFDM symbol (i.e., on a time axis).

The above-mentioned codeblock segmentation method and the datatransmitter method using the same will hereinafter be described indetail. For the better understanding of the present invention, a dataprocessing step of OFDM-based transmission/receivers will also bedescribed in detail.

FIG. 24A is a block diagram illustrating a data processing method of atransmitter according to an OFDMA scheme. FIG. 24B is a block diagramillustrating a data processing method of a receiver according to anOFDMA scheme.

Referring to FIG. 24A, the transmitter end performs a modulation or asymbol mapping (also called a constellation mapping) on a bit stream foreach user according to a QPSK (Quadrature Phase Shift Keying)-, 16 QAM(Quadrature Amplitude Modulation)-, or 64 QAM-modulation scheme at stepS11. By this symbol mapping, at least two bits are mapped to one symbol.

The bit stream is mapped to a data symbol. This data symbol is convertedinto a parallel data symbol by a S/P (Serial/Parallel) converter at stepS12. By the S/P conversion, the data symbol is converted into parallelsymbols as many as the number of sub-carriers allocated to each user(n). As shown in FIG. 24A, a data symbol of a first user 1 is convertedinto parallel symbols as many as the number (Nu(1)) of sub-carriersallocated to the first user 1. The sub-carriers allocated to individualusers (n) may be equal to each other or be different from each other, sothat the data symbols of the individual may be converted into the sameor different numbers of parallel symbols. In this case, the differentnumbers of parallel symbols are denoted by Nu(n).

The parallel data symbols for a specific user are mapped to Nu(n)sub-carriers assigned to a n-th user from among all Nc sub-carriers, andthe remaining Nc−(Nu(n)) sub-carriers are mapped to data symbols ofother users at step S13. By a symbol-to-subcarrier mapping module, thesub-carrier to which no user is allocated is filled with “0”, i.e., azero-padding. The output of the symbol-to-subcarrier mapping module isconverted into time-domain signals by an Nc-point IFFT (Inverse FastFourier Transform) module at step S14.

A cyclic prefix (CP) is inserted into the OFDM symbol generated from theabove-mentioned IFFT module to reduce an inter-symbol interference (ISI)at step S15. The CP-inserted OFDM symbols are converted into serialsymbols by a parallel-to-serial converter at step S16, and then theserial symbols are transmitted to a receiver.

Referring to FIG. 24B, the data processing method of the receiveraccording to the OFDMA scheme is performed in reverse order of a dataprocessing method of the transmitter. The received data symbols pass theS/P converter and the Nc-point FFT module, and then asubcarrier-to-symbol mapping process is applied to the resultant datasymbols. Parallel symbols are converted into serial symbols, aredemapped, and a bit stream is generated.

The above-mentioned turbo-code from among a variety of channel-codingmethods will hereinafter be described.

A turbo-encoder includes two encoders (i.e., a constituent encoder and arecursive systematic convolution encoder) and an interleaver. Theinterleaver is adapted to facilitate the parallel decoding of theturbo-code, and is a kind of Quadratic Polynomial Permutation (QPP).This QPP interleaver has defined the size of only a specific data block.The larger the data-block size, the higher the turbo-code performance.

However, an actual communication system divides a predetermined-sizeddata block (e.g., a transport block) into several smaller-sized datablocks, and then performs coding on the smaller-sized data blocks, sothat it is really convenient to implement the actual communicationsystem. These smaller-sized data blocks are called codeblocks. In otherwords, a single codeword with a long length is divided into severalcodeblocks. Generally, a single unit coded by both the CRC and theforward error correction code is called a codeword. However, the term“codeword” of the present invention indicates a data unit. This dataunit is acquired when the CRC-added transport block is channel-encoded.Therefore, if the size of a single transport block is larger than areference value, and is then segmented into two or more codeblocks, allcodeblocks are channel-encoded so that a single codeword is made.

Generally, the codeblocks have the same size. But, due to the limitationin the size of the QPP interleaver, one codeblock from among severalcodeblocks may have a different size. The forward error correctioncoding process is performed in units of a codeblock, and theinterleaving is also performed on the resultant data, so that theinfluence of a burst error during a RF-channel transmission can bereduced.

Thereafter, the resultant data is mapped to actual radio resources, andthen transmitted to a destination. Since the amount of radio resourcesused for actual transmission is constant, the rate matching for theencoded codeblocks is needed. Generally, the rate matching is performedby a puncturing or repetition process. The rate matching may beperformed in units of an encoded codeblock as in the 3GPP WCDMA system.The systematic part and the parity part of the encoded codeblock areseparated from each other, and the rate matching may be performed oneach of them.

FIG. 25 is a conceptual diagram illustrating a method for separating asystematic part and a parity part of a coded code-block from each other,and performing a rate-matching on the separated parts.

Referring to FIG. 25, a circular buffer may perform the rate matchingaccording to a transmission start position and the size of data to betransmitted. In FIG. 25, it is assumed that a code rate is ⅓.

The MIMO communication system based on the spatial multiplexing uses theSCW (Single Codeword) method and the MCW (Multiple Codewords) method.The SCW method indicates that a single codeword is transmitted viaseveral Tx datastreams, and the MCW method indicates that one or morecodewords are transmitted.

FIGS. 26A˜26B are conceptual diagrams illustrating a single codeword(SCW) and multiple codewords (MCW), respectively. A hybrid of the SCWand MCW method may also be made available. For example, provided thatfour Tx antennas and four Rx antennas are used, only two codewords maybe used. In this case, two SCWs transmitting two datastreams areinterconnected so that the MCW system is configured.

FIG. 27 shows a coding chain used for a HS-DSCH of a WCDMA systemaccording to the present invention.

If the spatial multiplexing is used, a maximum of 2 datastreams can betransmitted, and the streams are transmitted via the MCW. A coding chainof a first case in which a single stream is transmitted is equal to thatof a second case in which two streams are transmitted.

If a wireless communication system transmits data via multiple Txantennas, a data processing method for effectively segmenting a singlecodeword into two or more layers will hereinafter be described.

In order to reduce the influence of fading in the wireless communicationsystem, the present invention provides a data processing method fortransmitting data via the multiple Tx antennas, and a detaileddescription thereof will hereinafter be described.

The fading is one of the principal reasons causing a deterioration ofperformance of the wireless communication system. The channel gain valueis changed with time, frequency, and space. The lower the channel gain,the lower the performance. A diversity method used as one of solutionsfor the fading phenomenon uses the fact that there is a low probabilitythat all independent channels have low gain values. Generally, thelonger the time-, frequency-, or space-distance, the higher theindependency of a correlation of a channel gain value between two pointson time, frequency, or space. Therefore, in order to solve the fadingproblem, the codeblock-encoded bits are arranged to be evenly dispersedin a time-, frequency-, or space-domain, so that they acquire a highergain caused by the diversity.

The following embodiments will disclose examples in which inventivefeatures of the present invention are applied to an Evolved UniversalMobile Telecommunications System (E-UMTS). The E-UMTS may also be calleda Long Term Evolution (LTE) system. The technical specifications of theUMTS or E-UMTS have been prescribed in Releases 7 and 8 of TechnicalSpecification Group Radio Access Network of the 3rd GenerationPartnership Project (GPP).

FIG. 28 shows a downlink Frequency Division Duplex (FDD) sub-framestructure of an LTE system according to the present invention.

Referring to FIG. 28, a single subframe has a short length of 1 ms, sothat the degree of a channel variation on a time axis is low. But, amaximum of 20 MHz may be used on a frequency axis, so that a channelvariation on the frequency axis is high. Channels may be independent ofeach other on a space axis, so that they are evenly distributed on thefrequency- and space-axes to gain a diversity gain. The above-mentionedembodiments of the present invention may be applied to not only the FDDsystem but also a Time Division Duplex (TDD) system having a subframedifferent from that of the FDD system.

FIGS. 29A˜29B show transmission-chain structures of an LTE systemaccording to the present invention. FIG. 29A is a transmission structureof a Rank 3 equipped with three layers, and FIG. 29B is a transmissionstructure of a Rank 4 equipped with four layers.

In association with embodiments for effectively reducing thecodeword-to-layer mapping combinations, the structure of FIG. 29A maycorrespond to a third combination of Table 28, and the structure of FIG.29B may correspond to a fourth combination of Table 28.

The term “transport block” has been widely used for the UMTS or E-UMTSsystem, and is a basic data unit exchanged via a transport channel. Afirst transport block TB1 of FIG. 29A undergoes a data processing stepof a transmission chain, so that it is connected to a single stream(i.e., a single layer). In other words, the CRC is added to a singletransport block by the CRC attachment algorithm, and is channel-encoded,so that the channel-encoded result is allocated to a single layer. Theterm “transport block” has been widely used for the UMTS or E-UMTSsystem, and is a basic data unit exchanged via a transport channel. Thechannel encoding may be performed by a turbo-code or Low Density ParityCheck (LDPC) code.

In the case of a second transport block (TB2) of FIG. 29A, and first andsecond transport blocks (TB3 and TB4) of FIG. 29B, a single transportblock is connected to two streams (i.e., two layers). If the size of thesingle transport block is larger than a predetermined value, the singletransport block is segmented into several codeblocks (CBs). In thiscase, the CRC may be attached in units of a codeblock or transportblock. Also, if required, the CRC-attached transport block is segmentedinto several codeblocks, and the CRC may be re-attached to eachcodeblock. The channel encoding is performed in units of a codeblock.When the channel-encoded codeblocks are allocated to each layer, a dataprocessing step considering the spatial diversity is needed. The symbolstreams allocated to each layer is pre-coded for multiple antennatransmission, so that they are transmitted to a receiver via multiplexTx antennas.

A variety of embodiments associated with the data processing step beforedata is allocated to each layer after the channel encoding of FIGS. 29Aand 29B will hereinafter be described.

FIG. 30 shows a transmission-chain structure according to one embodimentof the present invention.

Referring to FIG. 30, each channel-encoded codeblock CB1 or CB2 includesa systematic part and a parity part. A rate matching module 81 performsthe rate matching on the channel-encoded codeblocks. The rate matchingprocess indicates that the sizes of the channel-encoded codeblocks arematched with a predetermined value. For example, a transmission startposition is controlled by a circular buffer of FIG. 25, so that the sizeof a codeword to be transmitted can be adjusted. The rate matching maybe performed for each channel-encoded codeblock or may also be performedon an overall part in which all codeblocks are interconnected.

The spatial division module 82 divides the rate-matched bitstream intotwo bitstreams, and outputs the two bitstreams. In this case, the orderof individual bits in the bitstream is unchangeable. The number ofdivided bitstreams is equal to the number of layers. The number oflayers in FIG. 30 is 2.

Divided bitstreams are applied to the symbol mapping modules 83 a and 83b. Each symbol mapping module 83 a or 83 b performs the symbol mappingon the received bitstreams, and outputs a symbol sequence. In order toperform the symbol mapping, a QPSK (Quadrature Phase Shift Keying),16QAM (Quadrature Amplitude Modulation), or 64QAM method may be used,but it should be noted that the scope of the present invention is notlimited to the above-mentioned methods, and can also be applied to othermethods as necessary.

Output symbol streams of each symbol mapping module 83 a or 83 b isapplied to each interleaver 84 a or 84 b. Each interleaver 84 a or 84 bperforms interleaving on each symbol stream, so that the order ofsymbols is rearranged. It is preferable that the interleaving be set toan OFDM-symbol-based interleaving. The OFDM-symbol-based interleavingindicates that a symbol assigned to a sub-carrier is interleaved withina single OFDM symbol. The order of symbols assigned to the sub-carriersis rearranged by the OFDM-symbol-based interleaving. The individualsymbol streams interleaved by each interleavers 84 a and 84 b areassigned to individual layers, respectively.

FIG. 31A shows a transmission-chain structure according to anotherembodiment of the present invention.

Referring to FIG. 31A, a rate matching module 91 performs the ratematching on the channel-encoded codeblocks CB1 and CB2. A symbol mappingmodule 92 performs the symbol mapping on a bitstream generated from therate matching module 91 and outputs a symbol stream. Detaileddescriptions of the above-mentioned rate matching and theabove-mentioned symbol mapping have been described in FIG. 30, so thatthey will herein be omitted.

An interleaver 93 receives the symbol stream from the symbol mappingmodule 92, performs the interleaving on the received symbol stream, andrearranges the order of symbols. Preferably, the interleaver 93 mayperform the interleaving on symbols corresponding to the codeblocks CB1and CB2, so that the symbols are evenly mixed. In other words, a symbolstream to which a first channel-encoded codeblock (CB1) is mapped, andthe other symbol stream to which a second codeblock (CB2) is mapped areevenly mixed by the interleaving shown in FIG. 31A (b). Preferably, therearrangement order of symbols by the interleaving may be predeterminedby a given algorithm, and the OFDM-symbol-based interleaving may beperformed on the symbols as shown in FIG. 30.

A spatial division module 94 divides an output symbol stream of theinterleaver 93 into several streams according to the number of layers,and outputs the divided streams. The divided streams are allocated toindividual layers. In FIG. 31A, the interleaver 93 and the spatialdivision module 94 are physically spaced apart from each other, but theymay be integrated in one unit as necessary. In other words, theinterleaver 93 performs the interleaving, and divides Rx data intoseveral symbols streams, so that the individual symbol streams may alsobe allocated to individual layers.

FIG. 31B shows a transmission-chain structure according to still anotherembodiment of the present invention.

Referring to FIG. 31B, a data bitstream is rate-matched by a ratematching module 95, and the rate-matched bitstream is applied to abit-level interleaver 96. The bit-level interleaver 96 performs theinterleaving on the Rx data bitstream. Preferably, the interleaving maybe performed in units of a bit group equipped with at least one bit. Thenumber of bits contained in each bit group is equal to the number ofbits mapped to a single symbol in each symbol mapping module 98 a or 98b. For example, if the BPSK scheme is used as the symbol mapping methodin the first or second symbol mapping module 98 a or 98 b, each bitgroup includes a single bit. If the QPSK scheme is used as the symbolmapping method, each bit group includes two bits. If the 16QAM scheme isused as the symbol mapping method, each bit group includes 4 bits. FIG.31B shows the case in which the QPSK scheme is used as the symbolmapping method. The data bitstream written in a row direction by thebit-level interleaver 96 is read in a column direction on the basis of abit group composed of two bits, and is then outputted.

The data bitstream generated from the bit-level interleaver 96 isdivided into several units according to the number of layers by thespatial division module 97. The first and second symbol mapping modules98 a and 98 b perform the symbol mapping on the data bitstreams dividedby the spatial division module 97. FIG. 31B shows the case in which theQPSK is used as the symbol mapping method, so that two bits are mappedto a single symbol. In FIG. 31B, the order of the spatial divisioncaused by the spatial division module 97 and the symbol mapping causedby the first and second symbol mapping modules 98 a and 98 b may bechanged to another order. In other words, the symbol mapping is firstlyperformed on the datastream generated from the bit-level interleaver 96,and then the symbol streams are segmented according to the number oflayers.

The individual symbol streams generated from the first and second symbolmapping modules 98 a and 98 b of FIG. 31B are equal to those of thespatial division module 94 of FIG. 31A.

An OFDM Symbol (OS)-based interleaver 93 of FIG. 31A performs theinterleaving at a symbol level, and a bit-level interleaver 96 of FIG.31B performs the interleaving at a bit level. However, the bit-levelinterleaver 96 performs the interleaving in units of a bit groupcomposed of bits corresponding to the symbol mapping method, so that thebit-level interleaving has the result equivalent to that of thesymbol-level interleaving. And, although the spatial division module 97of FIG. 31B is located after a single symbol mapping module as shown inFIG. 31A, the same equivalent effect is made.

FIG. 32A shows a transmission-chain structure according to still anotherembodiment of the present invention.

Although the structure of FIG. 32A is similar to that of FIG. 31A, itshould be noted that the spatial division module 94 of FIG. 31A isreplaced with the spatial distribution module 104. The spatialdistribution module 104 divides the symbol streams generated from theinterleaver 103 according to the number of layers, and at the same timerearranges the order of symbols. Namely, in FIG. 32A (b), the order ofsymbols corresponding to individual codeblocks is rearranged accordingto a given interleaving algorithm by the interleaver 103, and is dividedinto several symbol streams by the spatial distribution module 104, sothat the order of symbols is readjusted by a predetermined scheme. Inthis case, it is considered that the spatial distribution module 104performs the spatial interleaving. For example, a single symbol streamis configured by even-th symbols from among all symbol streamscorresponding to the individual codeblocks, and the other symbol streammay be configured by odd-th symbols. The method for re-adjusting theorder of symbols by the spatial distribution module 104 may be freelydetermined within the scope of maximizing the spatial diversity effect.In FIG. 32A, in the case of an actual system implementation, theinterleaver 103 and the spatial distribution module 104 may beintegrated in one unit as necessary. Detailed descriptions of the ratematching module 101, the symbol mapping module 102, and the interleaver103 are equal to those of FIG. 31.

FIG. 32B shows a transmission-chain structure according to still anotherembodiment of the present invention.

Compared with FIG. 32A, the embodiment of FIG. 32B controls a bit-levelinterleaver 106 to perform the interleaving on the basis of a bit groupcomposed of at least one bit according to the symbol mapping method usedfor the first and second symbol mapping modules 108 a and 108 b in thesame manner as in the embodiment of FIG. 31b . If the data bitstreamsdepending on the number of layers is spatially distributed by thespatial distribution module 107, the data bitstreams must be distributedon the basis of a bit group used for the bit-level interleaver 106. Inthe case of an actual implementation, the bit-level interleaver 106 andthe spatial distribution module 107 may be integrated with each other asnecessary.

In FIG. 32B, the order of the spatial distribution caused by the spatialdistribution module 107 and the symbol-mapping caused by the first andsecond symbol mapping modules 108 a and 108 b may be changed to anotherorder. In other words, although the spatial distribution module 107 ofFIG. 32B is located after a single symbol mapping module as shown inFIG. 32A, the same equivalent effect is made.

FIG. 33 shows a transmission-chain structure according to still anotherembodiment of the present invention.

Compared with FIG. 32A, the order of a spatial distribution module 113and interleavers 114 a and 114 b in the embodiment of FIG. 33 isopposite to that of FIG. 32A. In other words, the symbol streamgenerated from the symbol mapping module 112 is divided into two symbolstreams by the spatial distribution module 113. In this case, thespatial distribution module 113 re-adjusts the order of symbols so thatthe divided symbol streams evenly include symbols corresponding to thecodeblocks CB1 and CB2, and at the same time divides the symbol streamsgenerated from the symbol mapping module 112. Each interleaver 114 a or114 b performs the interleaving on the symbol stream generated from thespatial distribution module 113, so that the symbols are rearranged. Thesymbol streams generated from the individual interleavers 114 a and 114b are allocated to individual layers. In the structure of FIG. 33, thespatial distribution module 113 and the interleaver 114 a and 114 b maybe integrated with each other in an actual implementation process.

The embodiments of FIGS. 30 and 33 may also perform the bit-basedinterleaving on the channel-encoded codeblocks, before performing thesymbol mapping process. In other words, the embodiments of FIGS. 30 and33 perform the bit-group-based interleaving on the channel-encodedcodeblocks before performing the symbol mapping process, so that theorder of bits contained in the codeblocks may be rearranged.

The OS-based interleaving may not be used in the above-mentionedembodiments of FIGS. 30 and 33. In this case, if the case in which twocodeblocks are contained in a single symbol is not allowed, thesymbol-based processing is easier than the bit-based processing. Forexample, if it is assumed that the rate-matched CB1 has the length of 10bits, and the CB2 has the length of 10 bits, and the CB1 and CB2 use the16QAM, a single symbol is configured at intervals of 4 bits, so that thelast 2 bits of the CB1 and the first 2 bits of the CB2 are contained ina single 16QAM symbol. If the above-mentioned assumption is not allowed,the length of the CB1 or CB2 must be restricted to an integer multipleof a modulation order. This case is equal to the other case in which theOS-based interleaving is used as an identity interleaving. Therefore,although the OS-based interleaving is not used, the symbol mappingmodule and the spatial division (or distribution) module proposed by thepresent invention can also be used without any change.

The above-mentioned descriptions have disclosed the method for reducingthe number of codeword-stream (or codeword-layer) mapping combinations,the layer mapping process, and the method for effectively transmittingdata. The above-mentioned embodiments can be readily understood andmodified by those skilled in the art in various ways according to theabove-mentioned principles. For example, the layer mapping method ofFIG. 29A or 29B may be performed by codeword-stream combinations shownin Table 28, so that input data can be channel-encoded, be modulatedaccording to a symbol mapping scheme, and be mapped to each layer (oreach stream). FIGS. 29A and 29B correspond to third and fourthcombinations shown in Table 28, as previously stated above.

If a single codeword is mapped to two layers according to third andfourth combinations of Table 28, individual modulation symbols may bealternately mapped to two layers by the spatial distribution module 113of FIG. 33. In other words, as shown in FIG. 33, an even-th symbol ismapped to a first layer 1, and an odd-th symbol is mapped to a secondlayer 2, so that a diversity gain is acquired. Needless to say, theorder of the even-th symbol and the odd-th symbol may be changed toanother order as necessary.

The above-mentioned functions may be performed by a microprocessor, acontroller, a micro-controller, or an Application Specific IntegratedCircuit (ASIC) based on given software or program codes. The design,development, and implementation of the above-mentioned codes may beeasily implemented by those skilled in the art.

It should be noted that most terminology disclosed in the presentinvention is defined in consideration of functions of the presentinvention, and can be differently determined according to intention ofthose skilled in the art or usual practices. Therefore, it is preferablethat the above-mentioned terminology be understood on the basis of allcontents disclosed in the present invention.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present inventionwithout departing from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

INDUSTRIAL APPLICABILITY

As apparent from the above description, although the above-mentionedembodiments have been disclosed on the basis of the 3GPP LTE, the scopeof the present invention is not limited to this 3GPP LTE, and can alsobe applied to other wireless communication systems based on the MIMOscheme.

The method for indicating a combination between a codeword and a streamaccording to one embodiment of the present invention can rationallylimit the number of all combinations between a codeword and a stream inconsideration of a variety of aspects, so that it can reduce the numberof bits of information indicating the number of all combinations betweena codeword and a stream. As for the above-mentioned aspects, theabove-mentioned method considers a maximum transmission rate of aspecific codeword, compares the possibility of use with the number ofcases indicating a corresponding combination, maintains a combinationavailable for retransmission, improves a decoding performance of areceiver using the Successive Interference Cancellation (SIC) method andconsiders a stream grouping based on an antenna grouping and considers auser's convenience in a multi-user MIMO communication system.

Therefore, the present invention indicates all codeword-streamcombinations, which are required for both an uplink and a downlink in aMIMO communication system, with less number of bits, thereby increasingthe efficiency of control information.

The present invention provides a method for mapping codeblocks accordingto layers in a MIMO communication system, transmitting the mappedcodeblocks, and additionally guaranteeing a spatial diversity gaincaused by the spatial multiplexing.

In the case where a single data block is divided into several codeblocksand the codeblocks are channel-encoded, the present invention gives eachcodeblock a sufficient spatial diversity by adding simple functions to atransmission chain.

Although the preferred embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

The invention claimed is:
 1. A layer mapping method for spatialmultiplexing in a Multi-Input Multi-Output (MIMO) system, the layermapping method comprising: generating codewords comprising a firstcodeword and a second codeword; mapping the codewords to two or morelayers according to one mapping combination from among a plurality ofmapping combinations; and precoding the two or more layers to betransmitted through multiple antennas, wherein the two or more layersare mapped to the multiple antennas based on a precoding matrix used forthe precoding, wherein the plurality of mapping combinations comprisefirst mapping combinations for one codeword and second mappingcombinations for multiple codewords, wherein the second mappingcombinations for multiple codewords are used for transmitting thegenerated first codeword and second codeword, and wherein the firstmapping combinations comprise all possible mapping combinations forretransmitting one of the first codeword and the second codeword whenthere is an indication that the one of the first codeword and the secondcodeword was not received.
 2. The layer mapping method according toclaim 1, wherein the first codeword is mapped to a single layer and thesecond codeword is mapped to two layers, when a number of the codewordsis two and three layers are employed by the MIMO system.
 3. The layermapping method according to claim 1, wherein the first codeword ismapped to two layers and the second codeword is mapped to another twolayers, when a number of the codewords is two and four layers areemployed by the MIMO system.
 4. The layer mapping method according toclaim 1, wherein three layers are employed by the MIMO system, andwherein the first codeword is mapped to a first layer from among thethree layers, and the second codeword is mapped to a second layer and athird layer from among the three layers.
 5. The layer mapping methodaccording to claim 1, wherein four layers are employed by the MIMOsystem, and wherein the first codeword is mapped to a first layer and asecond layer from among the four layers, and the second codeword ismapped to a third layer and a fourth layer from among the four layers.6. A transmitter performing layer mapping for spatial multiplexing in aMulti-Input Multi-Output (MIMO) system, the transmitter comprising: atleast one antenna; a mapper configured to map codewords comprising afirst codeword and a second codeword to two or more layers according toone mapping combination from among a plurality of mapping combinations;and a precoder configured to receive the layers and precode the layersto generate signals to be transmitted via the at least one antenna,wherein the precoder is further configured to map the layers to the atleast one antennas based on a precoding matrix used for the precoding,wherein the plurality of mapping combinations comprise first mappingcombinations for one codeword and second mapping combinations formultiple codewords, wherein the second mapping combinations for multiplecodewords are used for transmitting the generated first codeword andsecond codeword, and wherein the first mapping combinations comprise allpossible mapping combinations for retransmitting one of the firstcodeword and the second codeword when there is an indication that theone of the first codeword and the second codeword was not received. 7.The transmitter according to claim 6, wherein the first codeword ismapped to a single layer and the second codeword is mapped to twolayers, when a number of the codewords is two and three layers areemployed by the MIMO system.
 8. The transmitter according to claim 6,wherein the first codeword is mapped to two layers and the secondcodeword is mapped to another two layers, when a number of the codewordsis two and four layers are employed by the MIMO system.
 9. Thetransmitter according to claim 6, wherein three layers are employed bythe MIMO system, and wherein the first codeword is mapped to a firstlayer from among the three layers, and the second codeword is mapped toa second layer and a third layer from among the three layers.
 10. Thetransmitter according to claim 6, wherein four layers are employed bythe MIMO system, and wherein the first codeword is mapped to a firstlayer and a second layer from among the four layers, and the secondcodeword is mapped to a third layer and a fourth layer from among thefour layers.